If you mean under the serial number that C would stand for 30,000-39999 as in these were number by lots of 10,000 so no letter is 1-9999, a is 10,000 -19999, B is 20,000-29999 and C is 30,000-39999 d,well, is you get the idea.
If they get to the end of the alphabet they double up the letters as in aa, bb etc etc
In 1943 BYF (Mauser Oberndorf) started going 100,000 per SN batch all others remained 10K batches
The Nazi Eagle was a firing proof the others were inspector proofs. If it has different numbers the part originated at one maker and was shipped to another for finishing
The head inspector kept his proof number so you will see WaA63 start out at Mauser Oberndorf then move to the Brunn (Czechoslovakia) factory as one example
There is a book dedicated to Mauser stampings for all nations, helpful to insure it is legit
It's always cute when a fake stamp is used for a gun that used to have that WaA# but should be a different number for the later year. Most forgers are smarter than that