It is usually referred to as a "jig hole." I don't think we know its exact use, but it probably had something to do with a jig that held the frame during some part of the assembly process.

These seem to show up during the early 1950s (I own a few with the jig hole and all of them are from that period). We do not find them on the earliest postwar, fixed sight K frames, I can say that with quite a high degree of certainty, since I own more than a dozen of those and have inspected many hundreds more of them. The hole seems to be gone by the time model numbers were applied in the late 1950s (in my experience).

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Jack
SWCA #2475, SWHF #318