As the picture shows, this is a serial number plate from an American Armaments 37mm M3 Semi-Automatic Gun. That said, I have not been able to find any pictures of the gun or the ammunition. I have a 37mm AT Gun cartridge in my collection, but it's not the same round as that used in the M3 semi, the casing being shorter by around 30mm or so. Does anyone have any pix of the gun or the round? Thanks, Bruce.