27 CFR § 479.67 - Cancellation of stamp.

The person affixing to a Form 1 (Firearms) a “National Firearms Act” stamp shall cancel it by writing or stamping thereon, in ink, his initials, and the day, month and year, in such manner as to render it unfit for reuse. The cancellation shall not so deface the stamp as to prevent its denomination and genuineness from being readily determined.