The Germans didn't use a prefix letter, only a suffix on German guns. They did use a prefix on Radoms. It is a suffix letter and I am quite certain it is a "z". (The font used by Mauser for those letters is not the same as the conventional fonts for German script - I don't know where they got it.)
After 9999z, they went to 1aa, and so on.
I don't know why the Germans limited numbers to four digits; perhaps just to make inventory easier and make it easier for the soldier to remember. In a company, it is probably unlikely that there would be two identical numbers and if there were the suffix letter should resolve the difficulty.