The S&W Factory will more than likely repair it for free as this was a RECALL from the early 1980's. When I purchased mine new in 1983, I did send it back for the free recall "upgrade" although I never did have any issues with it falling out. Never the less, they did modify it and all has been well for 21 years. There is an M stamped in the yoke area.

PS: I have also seen this happen with NIB M66's as well. The Factory repaired those as well free of charge.