[Replacement stock? Butt plate doesn't say Marlin, either and grip cap doesn't have brass screw, like my other Marlins?]
AFAIK, original Marlin lever action rifles have the serial number stamped into the internal butt stock wood.
A lot of Marlin's were made with the plain black (hard rubber/plastic?) butt plate that has horizontal serrations for "traction", ILO the Marlin logo plate.
My totally original 1967 Marlin 336, which my best friend ordered from Marlin back then, doesn't have the brass PG cap screw either.