Individual baffles aren't suppressors.
A stack of baffles, or a monocore, by itself, isn't a suppressor.
A pipe, or even the outer sleeve, machined to fit a monocore, is not a suppressor.
Owning both the tube and the core (or baffles) IS a suppressor, even if they're disassembled.
The serial number must go on the externally visible component, therefore the tube is the serialized piece.
Clearly, an oil filter is not a suppressor.
Once you design or redesign it to be mounted on a firearm, however, it is.
That happens when you assemble it with the muzzle adapter. The muzzle adapter is externally visible, and so can be serialized, which makes repair of the baffle stack easier.
Remember, early suppressors with wipes needed frequent repair (like every 10-20 shots), so a user-replacable baffle stack is hardly unprecedented.
A QD muzzle adapter for a suppressor is not a component of the suppressor itself. It is not required for the suppressor to function, only to hold it in alignment.