Roughly put, the "flattop" is the prewar/early war small-ring hammer. The "camel hump" got added about when it was chambered for the 9mm parabellum round (the "Red 9") and stayed on with the wartime commercial and M712 versions. I can't remember offhand which is on the gun used for Han's blaster and I don't feel like looking at the moment. *lol*