Some guy probably brought it home from WW2 to show "to the guys". A lot of that sort of thing happened. Soliders in WW2 were allowed to bring home rifles they had captured (nothing automatic, bolt actions stuff) and plenty of other stuff come home as well.
Also, a lot of it was just left lying around on Gunnery Ranges all over the state. When the war ended, a lot of ranges were closed down or given over to the state. Some guy goes out hiking or something and picks it up. Not smart, but then again, Kensington is not known for having a high number of geniuses.