In no particular order...
I do enjoy chasing down the specific text of laws that pertain to a subject that I'm interested in. It has on occasion given me PROOF that I could do more with regard to firearms than I had been TOLD by so-called "experts" who hadn't read ALL of the law. Among other things, I was told by one of these "experts" that it was illegal to use a high capacity magazine in a post-ban gun, which simply isn't true, at least not by Federal law though some states may have such a restriction.
As for your 12 round HK magazine, if it doesn't have the LEO/GOV'T marking on it or a serial number, you're golden. No proof of its post-ban status is possible.
Technically, you are not even prohibited from owning something which merely carries the LEO/GOV'T ONLY marking. You CAN legally purchase a new AR, for example, with that marking on it if you can get the manufacturer to place it on the rifle for you, and of course the rifle must be configured as post-ban legal. There is no specific language in the law that prohibits owning an item that carries that marking on it purely for the marking itself. It WOULD be illegal to own an LEO magazine that carries a serial number which can be traced to a post-ban date of manufacture, though.
Oddly enough, there is also no reference in the law to DATE CODES stamped on magazines or even lower receivers. A USGI contract magazine with an obvious date code (as in 12/91) but lacking a serial number or the LEO/GOV'T ONLY marking is also technically legal to own as it is not possible to prove its illegal status by the letter of the law.
You break NO laws by putting more than 10 rounds into a magazine designed for 10 rounds in a different caliber unless you modify the magazine in such a way that it will no longer work in its original application.
That point could be stretched even to the extent that one of the super sized caliber conversions for AR's like the .458 SOCOM might use a 30 round sized magazine to hold 10 rounds. It would be legal to produce 10 round magazines for this specific caliber and yet it would not technically be illegal to take these 10 round .458 magazines, replace the follower, and use them as 30 round .223 magazines...as long as they would still function in the .458 rifle.
Strange but true. There are loopholes to be exploited yet!
CJ