Two things cause confusion with those guns. One is the presence of the assembly number, which is quite prominent and often taken for the serial number, even when the serial number is present. That can lead to errors in record keeping.
The other is what I mentioned, that military personnel sometimes "liberated" their guns (Shock!! Hhorror!! Would our gallant soldiers do that???) and ground or filed the butt markings and true serial number off. That left only the assembly number, and sometimes that was used to replace the serial number on the butt.
FWIW, I have seen two of those guns with the butt marking removed, so I have to think it was a fairly common practice.