I'm not sure. As of 1986, "silencer" is legally defined as:
Quote:
any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for
use in such assembly or fabrication.
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My worry is that the oil filter is the part that does the suppression, therefore it would seem to be the part that needs to be registered. The ATF's website also states that "replacement parts are 'silencers' as defined in 18 U.S.C. § 921(a)(24) that must be registered and transferred in accordance with the NFA and GCA."