922R: What is a US-manufactured Part?

[giggitygiggity]

With respect to 922R and the requirement to have 10 or less foreign-made parts (of a list of 20 parts), what constitutes a US-made part? Specifically, does stamping “US” on an originally foreign-made part now make it a US-made part (I’ve seen some internet “experts” claim this to be fact, although I am extremely dubious). How about shaving 1/8” off the end of an originally foreign-made trigger... by doing so, did I make a US part since I did it on US soil? Does every bit of a given part, including the raw steel/aluminum/plastic need to have come from within the US; for instance if I imported a block of aluminum from Canada and machined a trigger from it, does that make the trigger a US part or is it a foreign part since the aluminum came from Canada?

Please provide references and evidence instead of simply stating your personal opinions and suppositions.

I am not trying to circumvent the law in any way. I want to make sure that I am in compliance without adding unnecessary costs and complexities on myself.

Also, I ask this question since I am about to put together a VZ-61 SBR and pre-manufactured US-made parts are not readily available. As such, 922R compliance will need to be achieved through my own effort or by employing someone with machining skills.

Mods, please keep this in the legal section as 922R does not apply strictly to NFA items or any specific type of firearm. Thanks.