The square-butt grips will fit a round-butt frame, but there will be a bit of a gap where the SB grips don't follow the RB frame. If it bothers you, you could always whittle a piece of wood to fit in there.
Eagle Grips also makes some good-looking round-to-square conversion grips:
"Classic",
"Heritage" (checkered), and
"Heritage" (smooth).