I know of one 4" RB M13 that came that way from the factory. It was presented to my former partner when he was voted "Officer of the Year" by our FOP Lodge in 1986. He had always carried a SB M13 and I remember the presentation gun being an oddity with the round butt. I liked the combination well enough that I had a 4" HB installed on a 2" RB Colt Lawman MKIII.
Smith was making up unusual configurations for Lew Horton and Ashland Shooter's Supply around that time, so why not?