As above, during WWII the Germans assigned codes to arms production to hide who was making what.
For the P-38 it was:
ac--Walther. From the early war to the end.
byf--Mauser, until 1945, then they changed to SVW. Note the upper case letters.
cyq--Spreewerke.
To positively identify a P-38 you need the makers mark, the year it was made, the four digit serial number AND any "block code" letter that was stamped at the end of the serial number.
The P-38 was made in 10,000 gun "blocks" with the serial number running from 0001 to 9999, then the sequence started over again at 0001 only with an "a", as "0001 a", to "9999 a", then it started over again at "0001 b" and so on until the end of the year.
For this reason, with three makers, using the same four digit numbers and block system, there can be literally many dozens and dozens of P-38 pistols all with the same four digit number.
To positively identify the gun, as above you need the maker, year made, four digit serial, and any block code.
Only Spreewerke didn't use a year stamp.