I have a similar gun that lettered as a factory four inch round butt. Apparently a few were made around 1971.





As far as I can tell there doesn't seem to be any hard and fast rule regarding barrel / frame steps. These are all factory original guns and the top straps and barrel steps vary considerably. Pictured below are a Model 66 no dash 2 1/2 inch round butt, Model 66-3 three inch round butt, Model 19-2 four inch square butt, Model 19-3 four inch round butt, and a Model 19-7 four inch round butt. Although it isn't real clear in the photo the 66 no dash and the 19-2 square butt have the same frame, barrel step and the others are all different.