Not something I've ever seen. Had an RB 2" 64 briefly in the '80s, and have a 2" -4 RB now.
Not to stray too far, but my 64-5 started out as a 4" round-butt. I only needed to swap the barrel to 3". I also have a 4" 64-3, and that has the normal square butt and heavy barrel.