TITLE 18 > PART I > CHAPTER 44 > Sec. 922.
Sec. 922. - Unlawful acts
Except as provided in paragraph (2), it shall be unlawful for a person to [red]transfer or [b]possess[/b][/red] a large capacity ammunition feeding device.
Paragraph (1) shall not apply to the possession or transfer of any large capacity ammunition feeding device otherwise lawfully possessed on or before the date of the enactment of this subsection.
This subsection shall not apply to -
the manufacture for, transfer to, or possession by the United States or a department or agency of the United States or a State or a department, agency, or political subdivision of a State, or a transfer to or possession by a law enforcement officer employed by such an entity for purposes of law enforcement (whether on or off duty);
the transfer to a licensee under title I of the Atomic Energy Act of 1954 for purposes of establishing and maintaining an on-site physical protection system and security organization required by Federal law, or possession by an employee or contractor of such licensee on-site for such purposes or off-site for purposes of licensee-authorized training or transportation of nuclear materials;
the possession, by an individual who is retired from service with a law enforcement agency and is not otherwise prohibited from receiving ammunition, of a large capacity ammunition feeding device transferred to the individual by the agency upon such retirement; or
the manufacture, transfer, or possession of any large capacity ammunition feeding device by a licensed manufacturer or licensed importer for the purposes of testing or experimentation authorized by the Secretary.
If a person charged with violating paragraph (1) asserts that paragraph (1) does not apply to such person because of paragraph (2) or (3), [red]the [u]Government[/u] shall have the burden of proof to show that such paragraph (1) applies to such person.[/red] [blue]The lack of a serial number as described in section 923(i) of this title shall be a presumption that the large capacity ammunition feeding device is not subject to the prohibition of possession in paragraph (1).[/blue]
That's the law. ATF operates under the concept of "serial number" (which most mags don't have) as being either a [b]post-ban[/b] date stamp and/or a "Restricted..." warning.
A hypothetical person who came into possession of such magazines is violating the law. However, in about 11 months, this law is going away, and possession of a post-ban hi-cap mag will be legal regardless of markings. So the risk is: would this person keep the mags in his/her possession until the ban expires and risk being found with them by one of a very few people who would know enough to care, or do you get rid of them?
ATF generally doesn't have time for petty stuff like this unless you've "gotten on their radar" (ATF's or another federal agency) and they want to make an example of that person. And even then, they'd have to KNOW that the person possessed prohibited mags in the first place. ATF has little interest in "illegal AWs" in the hands of most people, and "illegal hi-caps" rate lower than that.
If these mags had been ADVERTISED as pre-ban mags, and had been sold at pre-ban prices, then that vendor would have screwed the buyer by knowningly giving him restricted mags. But that doesn't seem to be the case. Most average folks would have no idea.