The letter of the law regarding high cap, post-ban magazines specifically requires their status to be identified by a date stamp, and you have that. Hypothetically. So they would be definitely illegal for a civilian to own at this time.
Hypothetically, if they disappeared until after the ban sunsets in 9/04, after that day they would be legal to own, but if you chose to pursue that route, you would wish to take steps to ensure that they are cached in a location and manner that would ensure that they could not be proven to be yours if by some chance they were to turn up and be investigated.
Hypothetically, I would do something like this by using multiple layers of plastic bags, an element resistant sealable outer enclosure, a shovel, a GPS, a good memory, and a suitable piece of public land out in the boonies. And everything to be buried would be carefully cleaned and sterilized of fingerprints, etc. with the aid of cleaning fluids, gloves, etc.
Probably it would be better to return them for legal mags.
CJ