IIRC, the lack of any LEO/Gov/Military markings or date-stamping is supposed to be an affirmative defense that the mag in question is pre-ban, i.e. it cannot now be proved beyond a reasonable doubt that the posessor knowingly had a contraband post-ban full-capacity magazine. Of course, that argument doesn't fly if the mag in question is for a gun that was not produced/imported prior to 9/94.
It's kind of an open secret, but used to be a lot of GI's and other people who were visiting, say, Germany would fill duffle-bags with full-cap Glock/Sig/HK/Beretta/etc. magazines that they bought at places like Franconia-Jagd for the equivalent of $8-15 each, and bring 'em back for their buddies or to sell at gun shows or on-line. The GI pipeline, in particular, got closed down in the final months of the Clinton administration, though...