Possession of such a device modified to hold more than 10 rounds is a violation. Easy to prove since the original magazine was never manufactured to hold more than ten rounds even before the 1994 AWB!
18 U.S.C. Chapter 44
921(a)(31) The term 'large capacity ammunition feeding device' -
(A) means a magazine, belt, drum, feed strip, or similar device manufactures after the date of enactment of the Violent Crime Control and Law Enforcement Act of 1994 that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition: but
(B) does not include an attached tubular device designed to accept, and capable of operating only with .22 caliber rimfire ammunition.
§ 922(w)(1) Except as provided in paragraph (2), it shall be unlawful for a person to transfer or posses a large capacity ammunition feeding device.
(2) Paragraph (1) shall not apply to the possession or transfer of any large capacity ammunition feeding devices otherwise lawfully possessed on or before the date of the enactment of this subsection.
(3) This subsection shall not apply to --
(A) the manufacture for, transfer to, or possession by the United States or a department or agency of the United States or a department or a State or a department, agency or political subdivision of a State, or transfer to or possession by a law enforcement officer employed by such an entity for purposes of law enforcement (weather on or off duty);
(B) 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 or such licensee on-site for such purposes of off-site for purposes of licensee-authorized trainig or transportation of nuclear materials;
(C) 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
(D) 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.
(4) If a person charged with violating paragraph (1) asserts that paragraph (1) does not apply to such person because of paragraph (2) or (3), the Government shall have the burdon of proof to show that such paragraph (1) applies to such a person. The lack of a serial number as described in section 923(i) of title 18, United States Code, shall not be a presumption that the large capacity ammunition feeding device is not subject to the prohinition of possesion in paragraph (1).
§ 923(i) ... A large capacity ammuniiton feeding device manufactured after the date of the enactment of this sentence shall be identified by a serial number that clearly shows that the device was manufactured or imported after the effective date of this subsection, and such other identification as the Secretary may by regulation prescribe.