I noticed a gap at the bottom of the back strap with S&W 1980's factory stocks and older N-frames, like the one pictured. It seems common, because I have seen other guns for sale online with the same thing, and there is currently a set of grips in the classifieds showing the same thing when mounted.
Any idea what the reason might be (besides new grips on old gun)?
When that gun was made, they were still sanding the stocks to fit the grip frame prior to bluing.
Later frames and stocks were not fitted. That's why new guns don't have perfectly fitting stocks.
Personally, I wouldn't let it bother me. If it does you, use a spring wooden clothes pin as a donor to make a small wedge. Hit it with some stain to get the color close. Push into the gap. Bob