Some years ago I found this poppet-type lock screw in a junk box at a gun show. It has about 50% of a dark finish remaining, but it has no markings whatsoever, not even a punch mark. It looks like the one I've seen in photos of the non-returning valve type with carbon steel spring. It functions, however, like a normal self-closing spring type. I can only think of two explanations. First, that it is an early or prototype self-closing valve type made by SA; second, that it began life as a non-returning spring type, but was converted to the later type, if that's possible. Any insight will be greatly appreciated.

Thanks,
Kevin