I was in the shower earlier thinking the same thing. What do you do with your included overseas parts when building from a parts kit? keep them in a bag in your gear so that you can show an identical part for each you swapped in hopes that they believe you?
I figure if you have two slant brakes and one rifle that they will trust that you didn't track down a foreign one, but that you found a USA made part and that is the one you installed, provided they are identical and have no reason to not use the USA part. Likewise, if that works, what would keep someone from just keeping a bunch of USA parts on hand and saying he already did the swap, being the lazy bastard he might be?
ETA: WTF?? that slant brake at brownell's sure as F*ck isn't the same as the one that came on my polish kit!! That one looks like it's made for barrel lenght compliance or something!!