The attitude on war trophy weapons has changed a lot since WWII. Mostly because the regular infantry class rifles these days are assault rifles (and here the term is being used correctly), and so under US law they are machineguns, and not legal for private citizens, as they cannot be legally registered because the registry has been closed, since 1986.
SO, no bring back AKs, or any other select fire or full auto arm can be functional and brought into the US as a "war trophy", at least, not by any private citizen. I believe the DEWAT category of war trohy is still allowed to be approved.
What "no importer's stamp" means is that the gun was brought into the US before 1968. It may be a war bring back, or it may have been inported befrore 68 when the requirement for an importer's stamp went into effect.
All the lack of a stamp does is allow for the possibility that the gun was taken from the enemy's hands or supply stocks and kept by someone who brought it back to the states. Something like a picture, with the signed insciption "Luger #4321 taken from Obfw Schnauser by Cpl Mazzeli, 11/17/43" makes the gun worth a lot more, to some people. Or the registration papers from the military as a war trophy, again, extra value, to some people.
Guns with proof of history are often the "won in a poker game" often on the ship back. Still a valid piece of history, and worth more than the "imported more than 23 years after the end of the war" category, again, to some people.
How much more, and to whom varies widely. Personally, I have some marked and unmarked milsurps, the importers mark doesn't matter to me. I'm interested in the type more than the markings.
All else being equal (and it almost never is) bigger bullets tend to work better.