YES THEY WILL FIT AS WILL ANY SQUAREBUTT K-FRAME, BUT I LIKE THE FLAT BOTTOM K-FRAME MAGNAS ON THE 66S AND 19S.
The 2.5" model 66-1 is a round butt, the 4" and 6" are square butt. Usually the 2.5" came with Magna service stocks, and the longer ones with Targets.