Smith Wesson Model 66-4 Square or Round butt?

If it is a 4" or longer barrel model, most likely it is a square butt (98 or 99% chance). If it is a 3" or 2.5" barrel model, it is almost certainly a round butt model.

If you have any doubt, remove the grips and examine the back strap of the grip. If it curves inward at the base, making a smooth outline, it is a round butt.

If you have any questions, contact me and I'd be happy to answer. I love this revolver.