Could be a number of different thread types, but more than likely it’ll be BA, probably either 2 or 1. In rare instances, BSC. Sometimes, depending on the age of the gun….it can be proprietary. They didn’t use the best tooling back then, and tap and die’s would wear down, and the screwer would continue to use the worn tooling…so sometimes when making a new pin/screw…you re-tap the hole to properly match the newly cut screw/pin threads. Pretty common when making a new pin.