I agree, no doubt it's an end run around the sbr classification. I do see some comments here that can get you in trouble and would like to point out that you do not want a compatible stock in your possession or even on your property before there's no chance of a raid as the prosecutor will use a little angle called "constructive intent" which gets you the same penalty as actually owning the forbidden fruit itself.
Keep aware and out of trouble (I like you all (but not in a gay way)and want you to stay free) as they will be looking for examples in the run up to loosing control and power.
The following combinations should never be able to be tied to you under current law unless you want the hand wringing talking heads on the media taking about you by name with that so serious and somber voice with terrorist, machine gun, militia, or arsenal in big bold letters right behind them.
1. Full auto fire control parts and a compatible host weapon. (juries are gullible and believe experts, even the revenuers "experts")
2. Tubing, baffles, and an attachment device for a firearm. (see above comment)
3. A short barrel and a host weapon even if the barrel is a spare that has never been on the weapon. Don't worry about the prosecutor as they never get charged even when they assemble such an nfa weapon for the first time in front of witnesses, a judge, and have I mentioned some idiots in a box that can't stand you because their missing dancing with the stars?