It looks like a very rough copy of a french military training pistol called a MAS 50

Scroll down for pics of the cocking lever open:

http://www.littlegun.info/arme%20fra...%2050%20gb.htm

Looks too basic to be a prototype.

Probably a kids metalwork project from back in the day.

(ccdjg’s fantastic Encyclopaedia of spring powered air pistols comes to the rescue again)

Cheers,
Matt