my guess is somewhere probably between 1930 and 1940, but that's just a guess based on cap guns I've seen from that time period. after WW2, most were a zinc alloy
I had one like it when I was a kid, that would be 1940's. They fired a single cap that came in a flat sheet with perforations, as opposed to the roll caps that were for repeaters. The single shot caps were larger and louder.