What you are referring to are the so-called "PC" (plain clothes) grips. The bottoms of the grip panels were rounded to minimize a clothing imprint when the revolver was carried concealed. I am not sure exactly when those began, but I think sometime after model numbering started. I do not believe pre-Model 10s had them. I have seen both diamond and non-diamond PC grips, and I think I even saw a pair of N-frame PC grips once.
The factory introduced "Modified Magna" stocks (called PC Magnas by most of us) in 1952 at around C200XXX s/n range . . . well before Model numbering in the '57-'58 timeframe. (I believe C103XXX s/n is very early for PC magnas but the fit appears good enough that checking the s/n on the inside of the right grip panel is the only way to know for sure . . . never say never when it comes to S&W)
After their introduction, S&W did in fact produce both styles concurrently and "generally" equipped the fixed-sighted revolvers with the PC magnas and target-sighted with the square bottom magnas . . . although the buyer could specify either style for the same price.
Russ