I have one (shown below under a USAF Model 15). The database David refers to is mine and I have quite a few pictures and several factory letters on others. In most of the letters Roy mentions that they left the factory with the U.S. marking on the (smooth) back strap. In the one or two letters where he doesn't mention it I believe it to be an oversight. I know of 2-3 other Model 56 revolvers that have had the U.S. marking removed. This is quite common with Colt M1911 pistols, Aircrewman revolvers and others.
Shameless plug: I keep databases on all post-war S&W martial handguns, not just the Model 56, and welcome any and all information, letters, pictures, etc. on U.S. handguns. There is little information published and I hope to produce a book someday on post-war military handguns.
Regards,
Kevin Williams