This has been brought up a lot lately and a lot of wrong information circulated, so I went straight to my local ATFE agent and they checked it all out with Washington and the NFA. You buy a stripped never assembled lower. It is sold to you as a stripped lower, nothing else. YOU assemble it into a pistol. No need for special markings or any of that crap. My ATFE agent, Mary Jane Humphrey, Louisville KY, just contacted the NFA and got the skinny direct from them. If it has never been assembled, the way you assemble it is what it is. No need to mark “pistol” on it, but it would not hurt to document the build with pictures. Once made a pistol, you can change it to rifle configuration so long as you put the butt stock on ONLY with a barrel of 16" or more, and you can swap back to pistol, no problem. Just make sure you do not have the stock on with a less than 16" bbl. You must by a stripped lower, and not one stripped down from a rifle but a brand new lower. When it is sold to you on the 4473 it should be recorded as a "stripped lower." According to my agent, to record a never assembled stripped lower as either a pistol or a rifle, is incorrect as there is no way of knowing how it will be assembled, and by recording it on the 4473 as anything other than a stripped lower MAY be a violation.
If there are state regs, then you must follow them, but this is the info I have gotten from my agent and the NFA regarding federal law. And you can also assemble the pistol with a carbine buffer tube, it is NOT a stock unless you put the stock part on it (which would be illegal unless SBRed). That is also direct fro the NFA and my agent.
To be safe though, I would not recommend you having the CAR buffer tube on your pistol and also owning the sliding part separately. Just buy the buffer tube, don't disassemble a CAR stock.
I do not see a problem though if you owned the sliding part AND had a 16” barreled upper so you could throw on the 16” upper and the sliding part of the stock, but I would make sure I had both in my possession at all times. That would be no different than the TC Encore/Contender.