I don't necessarily disagree with you here but it is not really your responsibility as the end product consumer to back check every single supplier that any particular manufacturer uses back to raw materials. Does Kip only use titanium that was mined and refined in the US? Are you sure? Do you have documentation from each of his suppliers? Does he only use coatings that are manufactured in the US or are some of them shipped from Canada or elsewhere? Are you sure he isn't outsourcing the manufacturing to the Caymans? Did you watch him make your mag tube? Etc. How about Surefire or Freedom Fighter Tactical? (Not trying to pick on Kip or any other company or to insinuate that their products do not meet the requirements at all... particularly with Kip, I believe it is quite the opposite actually and they meet 922r requirements without question). At some point you have to trust that the manufacturer is responsible for what they advertise. Consumer protection laws are in place to protect end product purchasers from misrepresentations by US companies.
922r does not mandate that any replaced parts be stamped "made in USA". That just makes it easier to prove, should BATF agent Smith show up at your door or your side, that your weapon has met the requirements, assuming that you are the person who put the weapon together, not just the current owner. You can do that as well, perhaps not quite as easily, by providing documentation, if you are the assembler of the weapon not just the current owner, of the required parts' "made in the USA" status... receipts, manufacturer documentation, etc. If you are not the assembler of the weapon, it does not matter as it is not illegal to own... just to assemble.
At least the last run were not stamped "made in USA" but based on ProMag's advertisement of it being made in the US it will not matter (being stamped "made in USA" is again not a 922r requirement). You would have made your purchase of this item based on the manufacturer's advertisement of "made in the USA" in good faith and ProMag would be on the hook for their misrepresentation should it be determined that it does not fit the requirements, not you. That's why, at least in the US, we have consumer protection laws. Hassle? Perhaps. Are you liable should someone at the ATF determine ineligibility with 922r? No. Can you be refunded your money by ProMag Industries should someone at the ATF determine ineligibility with 922r? Absolutely. Is ProMag responsible for any loss you suffer should someone at the ATF determine ineligibility with 922r? Absolutely (although this may vary from state to state most that vary do so on the side of being more harsh for the company).
Is it easier to deal with potential issues if the the part is stamped "made in USA"? Absolutely. Are any of the posters wrong who have stated that the ProMag parts are made in China? Only ProMag really knows...
If you are worried about it that much, and still want a c-stock, replace another 922r compliant inexpensive part...
DISCLAIMER: YMMV. Check with your own attorney and the BATF yourself. This is not construed as legal advise... You are responsible for anything you do... I like Butter Pecan Ice Cream even if you don't.... etc... etc... etc...