I’d say you lucked into one that got left in a corner, unfound for a while, maybe. I got a used one in 1986 from the era as yours with Chinese marks. Went bang every time. Ammo for it was cheaper back then, though...
6 posted on 02/23/2017 6:47:46 PM PST by W.
(How sweet it is to say "Former President 0bama!")