I've never seen a M-56 in the flesh, er, ah, steel either.

As for the grooved/non-grooved backstrap, Roy Jinks' book "The History Of Smith & Wesson" has 2 photos of one, next to a 2 inch Model 15 to illustrate differences, one of which was taken from above. You can clearly see the backstrap of the M-56 is smooth, as is the rear sight body, while both are grooved on the M-15. The front sight base runs from the frame/barrel junction forward, and it is smooth, but the base is also smooth on the M-15, as well. The U.S. marking on the backstap isn't visible in the photo, possibly because of the angle the photo was taken and the curvature of the round butt backstrap.