For some reason, this was not uncommon on S&W stainless revolvers of the early 70s, and sometimes later. The thumb-piece lugs of my 60, 67 and 64, all from the early 70s, would loosen, as well as the top strap screw of the 67. Nail polish did not work; my gunsmith secured all for me. My 640 in .38, decades younger, also required his services.

Kaaskop49
Shield #5103