Listed 1st in catalogue from 1950 

With primary C/D scales nearest to outer rim this layout offered great precision (scale length 28cm) over more popular 200 R with Trig scales next to outer rim +  H alongside central nameplate probably = country code for Holland

With primary C/D scales nearest to outer rim this layout offered great precision (scale length 28cm) over more popular 200 R with Trig scales next to outer rim

Double cursor version of innovative but impractical single long-scale 6-spiral patented (CH265561) design + tables of conversion factors in lid - try out an online simulation created by Andries de Man

Single cursor version of Innovative but impractical single long-scale 6-spiral patented (CH265561) design  + tables of conversion factors in lid - try out an online simulation created by Andries de Man

​Table of conversion factors in lid  + Dutch retailer Bladergroen was an stationer from 1903 to 1980 when it was bought out & name changed to Berlage  - try out an online simulation created by Andries de Man

One of four metal box finishes (2 x metal & 2 painted) occasionally used

Code H alongside central nameplate probably = country code for Holland + table of conversion factors in lid

The use of two-tone paper discs is probably for better contrast/legibility - table of conversion factors in lid

One of four metal box finishes (2 x metal & 2 painted) occasionally used

One of four metal box finishes (2 x metal & 2 painted) occasionally used

Conversion chart (fractions/decimals) in lid

Reason for non-standard green/turquoise paper discs & for polished inside box is unknown

​Code H alongside central nameplate probably = country code for Holland + ​conversion chart (fractions/decimals) in lid

Because of a shortage of aluminium during WWII the metal boxes looked identical but for a short time were made of much heavier zinc + code H alongside central nameplate probably = country code for Holland + ​conversion chart (fractions/decimals) in lid

Unconventional 200 R with black heavy-duty metal back plate instead of desk stand - reminiscence of military model with just a back plate  

RL = Rietz Liniaal (ruler) + failed project to make white plastic linear model + 2 windows with hairline, two expansion slots & table of constants for metals on back - part of prototype batch of +/- 50  

RL = Rietz Liniaal (ruler) + later (CI scale no longer red) improved version with self-lubricating bearings + 2 windows with hairline, two expansion slots & table of constants for metals on back

Version without a P scale + table of conversion factors in lid - subsequently succeeded by 400 D

Version with added P scale + table of conversion factors in lid - subsequently succeeded by 400 D 

Successor to 300 D with many more gauge marks & CI replacing S&T & P in lid + table of conversion factors in lid ​- see also larger sibling

See also smaller sibling - conversion table of fractions also on back 

Ng = Normaal (normal) grafici (printers) ​+ scales on back convert mm into cicero’s/didot points

Nmmi = Normaal (normal) mm to inches + scales on back convert metric mm into imperial inches  

N = Normaal (normal) & is possibly successor to Ha.A2 but with smaller font to make more room for advertising + see also larger sibling 

N = Normaal (normal) + personally enhanced (lighter plastic outer baseplate & peripheral cursor hairlines) model of last ALRO Technical Director Han Wanders - see also smaller sibling 

N = Normaal (normal) & is possibly successor to Ha.A2 but with smaller font to make more room for advertising such as Belgium photographers Vergouts + tables of conversion factors from 1010 Commercial in lid

E = Electro & successor to ElA - both rare circular examples for this type 

H = High Frequency with special scales for inductance, capacitance & resonance + CJD monogram for company co-founder Dussel indicates an early model + scales in lid provide the dB figure for various power & voltage ratios - probably inspiration for the much later modern plastic Decibel PTT models  

De Muiderkring = the Dutch publishing arm of Amroh (American Radio House) specialising in radio technology + pictogram colour coding & table of resistances for condensers in lid - see also waistcoat sibling

De Muiderkring = the Dutch publishing arm of Amroh (American Radio House) - see also larger sibling 

 AL-RO model designation (never repeated) is a reference to the full company name + CJD monogram for company co-founder Dussel & earliest pre-production model from original Wildhoeflaan 21 address in The Hague + imperial conversion factors in lid - metal box maybe a replacement   

Model designation clearly pre-empting soon to be granted international patents + early pre-production & synonymous with Ha (= Handel -= Retail) + imperial conversion factors - try out an online simulation created by Andries de Man in lid 

Calculates the angular measurement in a spherical coordinate system or more the horizontal angle from a cardinal direction

Personally designed by company co-founder C.J. Dussel + BoB3 = possibly Beton (concrete) & part of alternative model numbering system + forerunner to later Beton version but with additional 70 kg/cm2 scale & extensions for extra strong cement + scales in  lid are for calculating no. & size of reinforcement bars

Chemical credence comes from over 140 chemical-related gauge marks + table of all atomic weights from 1948 in lid - see also online simulation by Andries de Man 

Patented design (BE492292) by J.L. Brouhon from 1949 for wood merchants, wood sawmills, etc + tables of conversion factors from 1010 Commercial in lid + price included surcharge over other Ø 13cm models - see also online simulation by Andries de Man 

ElA = Electrotechniek (electrical engineering) forerunner to later 600 E & part of alternative model numbering system - both rare circular examples for this type 

Specially commissioned 50 Ng for Gravura N.V.  a printer & illustrator based in Dutch town of The Hague

Ha Tex = Handel (business) Textiel (textiles) for textile-based calculations with UK pre-decimal £sd currency & yards

Ha Tex = Handel (business) Textiel (textiles) for textile based calculations with UK pre-decimal £sd currency & yards + tables of conversion factors from 1010 Commercial in lid - see also online simulation by Andries de Man

Ha.A2 possibly part of alternative model numbering system & predecessor to 500 N AC-1.11 but with larger font & 4 pre-decimal & imperial conversion scales in lid + prefix Ha = possibly Handel (retail)

Highly complex specialised layout of scales designed by Dr. A. Lips + facilitates 26 medical based calculations (metabolic-related on front & blood ph/gasses on  back) - read more … or try out  Andries de Man's two online simulations: (i) front & (ii) back 

Load adjuster (possibly pre-production) made for Dutch airline KLM who had 1st fleet in Europe of American-made Convair 240 passenger planes - see also earlier prototype sibling  

Load adjuster prototype made for Dutch airline KLM who had 1st fleet in Europe of American-made Convair 240 passenger planes + table of conversion factors in lid - see also later (possibly pre-production) sibling

Slide chart made for Dutch airline KLM (old style logo) for converting imperial & US gallons to lbs, kg, etc for range & loading calculations - see also all-metal early version & prototype

All-metal silver coloured slide chart made for Dutch airline KLM (old style logo) converting imperial & US gallons to lbs, kg, etc for range & loading calculations - see final production model

Prototype slide chart made for Dutch airline KLM (old style logo) converting imperial & US gallons to lbs, kg, etc for range & loading calculations + strange reference to Portugal's Lisbon Portela airport- see final production model

Desktop version of smaller GoA with high-precision goniometric layout (double length Trig scales) by van Roon + specially commissioned dead reckoning flight computer for Dutch airline KLM for calculating course & wind correction 

Slide chart for calculating steel forms needed to construct: (i) fixed & opening bridges & (ii) points & crossing for railways & tramways + KLOOS = F. Kloos en Zonen’s originally a Dutch ship builder that evolved into making all manner of heavy-duty steel constructions

Made for Dutch Army with scales calibrated in NATO Milliradian (Mils) ​for angular measurement + pouch with belt clip shows it was intended for use in field of action 

Made for Dutch Army with scales calibrated in NATO Milliradian (Mils) ​for angular measurement but being metal cased version was for army barracks or classroom/study 

Made foreign armies with scales calibrated in NATO Milliradian (Mils) ​for angular measurement but being metal cased version was for army barracks or classroom/study 

Ned. Octr. Aangevr. = Dutch patent applied for so possibly an early pre-production model with CJD monogram for company co-founder Dussel + imperial conversion factors in lid - sadly metal box & patent nameplate are replacements   

Larger versions made for PTT 

Specially commissioned by UK-based A.C. Wickman Ltd. + designed by Nestor Piret for wire drawing while working for defunct Dutch company Van Thiel's Draadindustrie + paper table in lid for Wichman Wimet Die sizes 4/10 to 36 showing equivalent sizes in circular Mils & mm according to 6 wire gauge standards  - try out an online simulation created by Andries de Man

PTT = Posterijen, Telegrafie en Telefonie or old Dutch post, telegraph & phone company & ​NL NR 04 - 2220 = ordering no. + see smaller sibling

PTT = Posterijen, Telegrafie en Telefonie or old Dutch post, telegraph & phone company & ​NL NR 04 = ordering no. + see larger sibling

Adapted 200 R with gauge marks as pay-out odds for 6 popular types of bets possible on the numbers part of a French style roulette table - reputedly conceived by Capt. Hans Feierabend  read more … 

Shell © clock (klok) design for calculating in 4 standard units viscosity of various oils at different temperatures - try out an online simulation created by Andries de Man

Rare load adjuster for an American helicopter + prototype possibly made for aviation branch of Dutch Navy (Marine Luchtvaartdienst - MLD) who nicknamed it Jezebel 

One of a pair of rules, without slides but with erasable cursors faces for hand-drawn pencil hairlines, made for the Dutch Army – the laminated (Alroderen™) paper tables & model # are ex-Army

Two of a pair of rules, without slides but with erasable cursors faces for hand-drawn pencil hairlines, made for the Dutch Army – the laminated (Alroderen™) paper tables & model # are ex-Army

Specially commissioned 10 R for T.K.F. = Twentsche Kabelfabriek (Twentsche Cable Factory) was originally a cable manufacturer based in Dutch town Haaksbergen but is now a leading supplier of connectivity solutions

Specially commissioned 10 R for Van Kranenburg a now defunct heavy machinery company based in Dutch city of Rotterdam

 CJD monogram of company co-founder Dussel suggest an early model for determining large/heavy goods vehicle (HGV) tyre sizes + 3rd party Table of Truck Tyre Pressures in lid

WeM is part of alternative model numbering system + WeM = possibly Werktuig- en Machinebouw (mechanical engineering/machine industry) + possible forerunner to later (?) 200 R

WeR = possibly WeRktuigbouw (mechanical engineering) & part of alternative model numbering system + forerunner to later 100 R - later WeR2's included a  version no. although only discernible difference on a later sibling was scale annotations in red   

 WeR = possibly WeRktuigbouw (mechanical engineering) & part of alternative model numbering system + forerunner to later 100 R - model suffix 5 is possibly version no. although only discernible difference to earlier sibling is scale annotations in red