The flaming bomb should be an Ordanance marking. The initial stamps are inspectors cartouches- there are books showing who the actual inspectors were in any specific installation. Verifiy authenticity if the price is inflated.
The serial number may have a US prefix/suffix rather than the United States Property stamp.
It would have U.S. with an Ordanance bomb on the right side receiver,
The barrel will have an ordinance bomb where it enters the receiver,
and it will be a cylinder bore.The stock will have a Crossed cannon. If I recall the riot gun will not be inletted for a rear swivel, but the trench gun stock will.
Mr Moore...respectfully, the only firearm that I can think of that is marked US Property is the M1911 pistols. The Garands,Springfields,Krags,M1917's and every other gun I can think of does NOT have US Property.