file off the serial numbers

From the practice of filing the serial numbers off of firearms in order to make their origin and ownership difficult to trace.

• "[…] that song belonged to Noisy when he got through filing off the serial numbers and changing the body lines. I heard it again, oh, must have been twenty, twenty-five years later, in a cabaret in Luna City. From Noisy. But he had changed it. Fixed up the scansion, given it a proper rhyme scheme, dolled up the tune."
• "For a while now, I've been pottering away at a Star Wars fanfic novella ([URL redacted], if you must know). I've received a few pokes about filing off the serial numbers and trying to sell it as an independent work."
• "I've heard a rumor that Bujold started the Barrayar series as Star Trek fanfic, and there's enough resemblance there to make that seem very plausible to me. Suppose she did. By filing off the serial numbers, she was able to make the universe her own, to change as she needed to."
• "I say “may have” because [E. L.] James has “filed off the serial numbers” of her fanfic."