I have to disagree with tdan on the issue of the square butt cambats from S&W beng less common than the round butt version. The square butt version was around for several years before the round butt version came out, and remained in production until S&W made the switch to all round butt frames.

You simply don't see them for sale as often because too many of us like them and won't sell them. On top of that, a lot of us hate round butt guns, and switched the factory round butt combats out for RB-SB conversion grips like those made by Hogue. Therefore there are more RB combats around for sale.

feral-
You can just see the backstrap on this 29-4 wearing this set of combats. These are dated March 1989 on the inside of the right panel. I know that these grips were available for at least 3 years before then though. These type grips were never numbered to any gun though.