I just finished populating a Green Mountain Tantal barrel with a matching numbers kit that has been on hold for years. The build went very well with all journals diameters matching the parts. I did have polish off the black finish to get the gas block to press on, but that was all.
There is still one more kit to build, the barrel is a duplicate Green Mountain barrel, so there shouldn't be an issue. I will report back as to how it shoots, but other builds shoot well with the same barrel.
To answer your question, the Polish barrels are proof marked with inspectors stamps. US barrels are just manufactured & sold blank.