Does it necessarily mean a gun was factory re-nickeled if under the grips is stamped R-N 1980? Or re-blued if it is stamped R-B and 1979?
Or could that mean something entirely different?

If these are factory stamps, they mean exactly what you wrote. A re-blued gun might also be stamped R-S for refinish-standard. I suppose these could be faked but I have not heard of this.

As I recall these would be stamped with a date code in smaller numbers, usually on the forward left lower grip frame and in mm/yy format - 1265 would be December 1965, for example. I have seen a few guns with six numbers, probably mm/dd/yy format. The date stamping ended sometime in the late 1970s, I think.

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Alan
SWCA LM 2023, SWHF 220