We're hearing reports that the ATF has been contacting persons who have purchased "solvent trap adapters" from eBay sellers and collecting the parts that they purchased. Most of these eBay sellers seem to have disappeared and it may be that the ATF has shut them down and is now contacting their customers.
The ATF takes the position that a suppressor part is a suppressor. Therefore, if you purchase one of these parts, the part may itself be considered a suppressor. The problem here is that while an approved Form 1 application does give you the right to manufacture your own suppressor, it does not give you the right to purchase a suppressor manufactured by someone else. So, if a solvent trap adapter is defined by the ATF as a suppressor part and a suppressor part is considered by itself to be a suppressor, then the purchase of the solvent trap adapter alone will be treated by the ATF as the purchase of a suppressor which requires an approved Form 4, not a Form 1.
The ATF seems to be moving towards using a person's intent while making the purchase in determining whether a violation of the NFA has occurred. So, even if you purchased a "solvent trap adapter", if you use it as a part of a oil filter suppressor, the ATF may argue that your intent was to make a suppressor and charge you with felony violation of the NFA. If you purchase a solvent trap adapter and never use it for any purpose other than cleaning your firearm, you're perfectly legal. This line of reasoning would also seem to apply to some of the build-your-own-suppressor kits available online. These may require a Form 4 instead of a Form 1.
While the ATF's position in making this argument may be legally questionable, it is not a fight that you want to engage in, as your legal defense could cost you tens of thousands of dollars, if not hundreds of thousands of dollars. That type of expense, hassle and stress is just not worth saving a few hundred dollars in doing it right.