The thing now is, the ATF is starting to look at the threaded "Solvent Trap" as a pre-made silencer part. Regardless of what your initial intent was when you purchased it.
You can not Form 1 from pre-made pieces, because they are, in themselves, a silencer. Also, no one other than SOT's can produce silencer parts, w/o a Form1. So "these guys" making the tubes and caps might be in big trouble.
You would, technically, need 2 stamps or an SOT to make a Form 1 suppressor from a "Solvent Trap".