US firms were not allowed to manufacture and sell hi-caps (more than 10 rounds) after the cut-off date. Any hi-caps manufactured in this country after that date must be stamped with the date and/or a statement to the effect that they are for LEO use only. Civilian possession of such a mag is illegal.
If a US importer had a contract in effect with a foreign manufacturer that was not fulfilled as of the cutoff date, the companies were allowed to fulfill the contract, even if that meant importing hi-caps manufactured overseas after the ban date. I am assuming that those contracts had to be filed with ATF, and I can't imagine that any of them extended mmore than a year or so past the ban.
Any hi-cap manufactured overseas after the ban date, excepting those exempted by contract, are illegal to import or own in the US. If you bought a SIG hi-cap on a trip to Germany, it would be smuggling for you to bring it back with you. If customs found it, they would assume that it was post-ban and therefore illegal. If you did bring it back and got it through customs, I'm not sure how ATF could tell that it was post-ban. I don't know if foreign manufacturers are cooperating with the US and marking or dating their post-ban production, since there is no such thing as pre- or post-ban in other countries. They would have to mark post-ban LEO mags imported into the US.