I'd view foreign proofmarks on a US-made commercial piece the same way that importers stamps are viewed on military surplus weapons - it lowers the value.
Using the M1 Garand as an example - from the collector's viewpoint there are three tiers:
Straight GI issue is the most highly valued - even though possibly mis-matched.
Import-stamped are less desireable, unless one has a spare barrel and maybe a stock.
ODCMP-build-ups are just 'parts guns', regardless of their Government connection, because their barrels and wood are new commercial production, and not wartime.
A collector wants the piece in as original a condition as is possible, so even the M1s that went through the postwar 'Clean and Repair Programs' are more desireable when they stayed in US hands, than those that went to 'Foreign Aid' and were re-imported.
The term 'No Import Markings' always brings a higher dollar.