Ref 508.901 Stopwatch Help

Hello everyone! I recently came across a ref 508.901 mil-s-14823 stopwatch and had some questions I was hoping to get help with. I've been doing some research and can really only find info on the 1976 version which is marked with an n.s.n number. This particular version has an f.s.n number, which I think means it had to be made prior to September 1974.

I saw a website say early versions were made prior to 1974 but cannot find real info on any from dates prior to 1974. I'd mainly like to know if it's rare and possibly if there's any info out there that others know. It works fully and isn't in horrible condition. Few minor wear marks on the case, and some scuffs on the lens but no cracks. Pictures provided below.