The legal definition of silencer includes "any part intended only for use in [a silencer]". About a year ago the ATF determined form 1 kits, undrilled cones, and other similar parts are intended only for use in making silencers. Thus, they are already silencers, and can't be used to make a silencer on a form 1.
Prior to last year's mass denial they didn't really care. Since then they've required extra information prior to approval, which they use to determine whether or not you're using parts that meet their interpretation of the definition of silencer. I'm sure there's people lying on their forms, getting approved, and using premade parts, but that's not something I'd recommend.