A lot (Probably Most) of the late/unknown codes are changes to the existing codes to hopefully confuse the allies as to what was being made where.
A commonly known example is Mauser Obedrndorf byf became SVW in about Dez 44, Spreewerk cyq became cvq and Walther ac became ?, just went blank.
This was not a first time thing for the Germans as they changed from number codes to letter codes in 1940.
Many of the late war code changes can be identified by checking the WaA numbers because they didn't change when the codes changed.
Sarge