A wierd one indeed..
-rear sight has not as wide notch as a it shoud be
-if a 42 mauser, the '42' seem to be oblique angled
-the toggle knobs as Ron mention. The holes also looks big..
-notch on the righ side of receiver near frame
-barrel looks thin for a 9mm
-stamping in general
Any chance you father was good with cast iron?