The CCCP cartouche is found only on rifles produced at Izhevsk. It may be a form final acceptance. It was probably used from day 1 all the way until the end of production. but not many factory original examples have survived so it is hard to be certain. Next to the CCCP cartouche, two cyrillic letters in a circle were stamped and are probably the initials of the inspecting official. The same two letters are found on the barrel. In 1938, it was "ГВ", and I have seen a matching 1941 M38 Finn with "ГП".
Sgt. Rock, can you read your 1939? Can you check the barrel and see if the same initials are stamped there?
The stock cartouch and "ГВ" inspections from my original '38 attached.