From the ATF Firearms Q&A
(O17) What evidence is sufficient for Federal firearms licensees to be sure
that particular semiautomatic assault weapons are "grandfathered" weapons
which are not subject to the restrictions on possession and transfer?
The requirement that semiautomatic assault weapons be marked "RESTRICTED
LAW ENFORCEMENT/GOVERNMENT USE ONLY" was not effective until July 5, 1995.
Thus, semiautomatic assault weapons manufactured from September 13, 1994-July
4, 1995, may not be marked with the restrictive markings. Additionally,
sporting weapons may have been modified after September 13, 1994, so that they
are semiautomatic assault weapons, e.g., a pistol grip and magazine extension
are installed on a sporting shotgun.
(The important part)
Licensees obtaining semiautomatic
assault weapons which do not have the restrictive marking should obtain from
the seller an invoice, bill of sale, or other documentation indicating that
the weapon in its present configuration was lawfully possessed on or before
September 13, 1994.
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[27 CFR 178.92(a)(2)]