Possession of suppressor parts without an approved form 1 is a no-no
Possession of all parts or possession of some parts? If I don't possess some parts how do I file a form 1?
In theory, those form 1 "kits" are not suppressors or even suppressor parts until you modify them to be suppressors. Usually by drilling the holes for the bullets to pass through. They are Solvent Traps or whatever until so modified. Consider that if there was any question of legality, the many companies selling them are taking a much bigger risk than the purchaser.
That said, you do not want to play semantics games with the ATF. Get the specs from whoever you are buying the kit from and make up a basic drawing with all the dimensions on it. Sometimes the vendor already has a drawing with the dimensions on it. Submit those dimensions with your form 1.
Only after you have your approved form 1 in hand, do you purchase any parts. Then immediately send the tube out for engraving. (remember it is still not an NFA item, but you now have approval to make it one) Only after your properly serial numbered and engraved tube is in hand do you then drill or machine your baffles and end caps to create your NFA item.
We can argue all day whether you
need to do it in that order or not, but that is the safest way.