I knew I had heard that name before. Nice catch.
Yeah I do not think this was done as a sniper rifle. I have the counts of those rifles and even before the war started they did not want a 30:06 Sniper rifle. They wanted a 7.62 as they were required to switch because of Nato. So only about a 100 Model 70's and M1D's went to NAM in 30:06. Then within about a year the 7.62 M40 arrived.
I interviewed LtCol Chandler who was in charge of the RTE shop during the Vietnam era. I asked if any armorers marked rifles in anyway so we could be able to tell them today. He said nothing was marked and he wouldn't have allowed it. He said it was a big personal beef of his that his rifles did not look like the Army rifles, with all their elctropencil markings and cartouches and all that. He said he thought they were ugly. He said he did not want his rifles marked in anyway.
My honest thoughts were many of those armorers continued to build rifles into the Civilian world. I really do not think USMC Property would be allowed be marked with the Name.
I would research if this Donovan continued to build rifles past his time in the Marines. That is sort of where my research would start if this was my rifle.
It's a beautiful rifle by the way!