SARs from Century have the grip as a US part. The issue is not how many US parts, but the number of foreign parts. Adding a US brake to a gun which did not have a brake to begin with, will not reduce the foreign count to allow the substitution of a foreign grip.
Not the thing that gets you busted at the range. But a local was driving to a shoot this summer, pulled over by a state hwy man for something else, arrested for 'machine guns' that were simple factory AR and AK semis, 7hrs in cuffs, guns missing from the evidence room, and an ongoing lawsuit.
In case the officer already has a case of the ass, keep your toys in order.