Grips are NOT interchangeable between square and round. You always need the right grip type for the metal shape underneath.
Back in the early 20th century, the "fashion" in revolver grips was to have the grip frame metal showing "all around" the edge, with wood side panels. It was a cosmetic "leftover" from the age of the Colt SAA, Remingtons, etc of the 19th Century.
Thus, if you wanted the two different shapes yet appeal to this asthetic, you had to have different metal shapes.
We got over that when the "Modern Technique" shooters realized that you need to "fill in" the area behind the triggerguard. This was accomplished with either grips or the Tyler T-Adapter. Eventually Hogue, realizing people would buy grips that cover part of the grip frame metal, did their "conversion grip" which makes a round-butt grip frame FEEL like a square butt.
S&W eventually dumped square-butt grip frame metal completely, to simplify the product line knowing that square-butt fanatics could always use a Hogue or equivelent grip.
