The jig hole seems to have been a "thing" in the late 1940s and early 1950s. It first seems to start showing up c. 1948 and continues into the mid-1950s. I have several early '50s K frames that have it, both fixed and adjustable sight models. I have one or two S prefix M&Ps that have it, and my database shows only some in the high S range on which the jig hole appears.

I'd have to check, but I don't recall any of my model numbered guns that have the jig hole. However, if I recall correctly, there is a K-22 (not marked Model 17) made in early 1957, that has it. When I get a chance, I'll pull that one out of the safe, remove the stocks and check.

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Jack
SWCA #2475, SWHF #318