A lot of folks deplore the import markings, especially of they are large and/or in a conspicuous place, but no milsurps could be imported without some mark, so the alternative is marked guns or no guns.
Those so-called "capture papers" were supposed to be given out for any and all captured enemy equipment that was going to be kept by the GI. There was no specific form; some were printed, but many were just typed and mimeographed.
I have seen papers like that for M1911A1 pistols, and M1 carbines, all fake. There are endless rumors and stories, but no officers legally set up tables and sold M1 rifles in a war zone, no soldiers legally brought back their issue weapon, General Patton did not hand out pistols or rifles in lieu of medals. But "I was given this rifle by Patton himself" sounds a lot better than "I got this rifle by courageously writing a check to DCM."