Given identicle guns to choose from,,one import marked and the other not,,the gun that is not import marked will generally be sold first and generally for a bit more than the marked one.
The marking can have a bigger effect in the pricing of military surplus firearms.
The location and size of the marking effects the value too as Muley Gil says.

The location of the marking I believe has to be in an area not covered by the stock, grips or other parts. That was not always the case and early on some were marked under the grips. Some were also marked with lettering so small it almost looked like a small dent to the naked eye. I had a couple of BHP's from an outfit in CA that were done that way.