Jpeelen, I think I missed your Geco box date codes when they were posted originally. I am very interested in these codes. Thank you very much for posting them.
Of course 41R6 IS 41RG. My mistake.
I have quite a few of the blue Geco boxes from the 50s which I got from a German friend. The codes are 7Gs (ctgs dated 58), 5Tc, 7Es (ctgs dated 55), 7Ts, H9 (ctgs dated 55), 7Ns (ctgs dated 55), s.s.G.S (black plastic blanks), S7 (black plastic blanks).
These codes do not seem to match the one you describe above. I recently sent the ECRA some work on the Geco RWS codes on pistol ammo from 1928-1945 that I have been trying to understand along with my best guess, based on a relatively few boxes, but some with dated ammunition, and it seems to be closer to your code, except mostly just two letters like KX, NC, PK, UX, VO, XT. This code seems to be different from either the one you list above and from the one from my 1950s Geco boxes.
When do you think these codes likely changed, for example, when was the one you described adopted?
Do you have any insight into the earlier codes???
Many thanks for the great insight ou provide.