I agree. I plan on lettering my model of 1917 as it was shipped to Lt. A.C. Trego at the Springfield Armory. Lt. Trego was in charge of the Ordnance Department overseeing production of all S&W model of 1917 Army DA .45 revolvers and it turns out that it was his personal gun. There is no United States Property marking, no US Army marking on the butt, no flaming bombs and no eagles or large "S" inspector stamps to be found.
There are also dozens of 1917's that were shipped out to the various arms manufacturers of the day for testing and ammo function tests. Sure would be nice to find one of those!!!
If I owned John Dillingers .45 DA, I would definitely letter that one too. Unfortunately, the FBI could not find any serial numbers on the gun as John was very careful about removing all markings of any kind.
Besides, Mr. Jinks derives income from all of those letters and he deserves and earns every penny.