The oil filter, which is the operational part.The oil filter or the threaded adapter?
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.
Replacement of any component or component parts of a registered silencer, other than silencer wipes, would be a violation of the NFA if performed by a non-licensed manufacturer.