“Groupe Operationnel de l'A.B.C.”

Data:

• Made by Nestler.
• Probably used by the military.
• Marked “Groupe Operationnel de l'A.B.C.”
• Abbreviations “K, Em, Ea, T 41, OPL, EPA, EJA, EPM, EJM”
• Instructions (translated):
  1°/ – Align the "distance" scale graduation corresponding to the average distance (for an EPA) or the true distance (for an EJA) with the red mark T 41 (or OPL). 2°/ – Locate the graduation on the "metric deviation" scale corresponding to the calculated EPM (or EJM).
  3°/ – Read the EPA (or EJA) on the "angular deviation" scale, opposite this graduation.
  

Question:

1. What do the abbreviations stand for?
   “O.P.L.” stands for Optique et Précision de Levallois, a French maker of optical instruments a.o. telemètres (range finders) from 1919 to 1964 Andries de Man
   There is an American “T-41” stereoscopic rangefinder which was used for the post-WW2 M-47 tank. Andries de Man
   “A.B.C.” could stand for “Arme Blindée et Cavalerie” (Armoured Cavalry) or “Atomique-Biologique-Chimique (NBC). I guess it is the first one. Andries de Man
   
   Albert Nestler Entfernungsmesser c1938
   gwr (i.e. Dennert & Pape), WW2
   Compare with Albert Nestler Entfernungsmesser c1938
   Ea = écart angulaire = Ablage in Winkelsekunden = angular deviation Em = écart métrique = Ablage in m = linear angular deviation D   = distance = Wahre E[ntfernung] = real distance K   = Gerätekonst. = instrument-dependent constant The current, French, slide rule has only two values of K and the German has seven (integer) values. The German one seems to use Em for a base length which, in combination with an optical magnification, leads to a value of K. Andries de Man
   ChatGPT suggests:

   Abbreviation	Meaning	English
   EPM	Écart Probable Métrique	probable linear deviation
   EPA	Écart Probable Angulaire	probable angular deviation
   EJM	Écart Juste Métrique	exact/true linear deviation
   EJA	Écart Juste Angulaire	exact/true angular deviation

   ChatGPT