Seems like you've got "mostly" good information in this thread. Dogtown tom's summary is the best so far. Nevertheless, I can't help but summarize my understanding of the state of the law as well...
Stripped lowers are transferred on the 4473 as "other," not as a pistol or rifle. There's no option to instruct the dealer to transfer it as something else, so if you run into a dealer who won't transfer a lower as "pistol," there's no problem. But the more important point is that the 4473 doesn't "register" your lower as a particular configuration. It simply describes its current state of being. If it's configured as a pistol, it's transferred as a pistol. If it's configured as a rifle, it's transferred as a rifle. If it's stripped, it's transferred as other.
(If, as I describe below, the gun in question was originally built as a pistol, then gets configured as a rifle, sold to another person through a dealer, transferred as a rifle on the 4473, and then the new owner makes it a pistol again, I believe this would be legal. And the ATF would have to prove a gun was not a pistol before it was a rifle to have a successful prosecution, so it's really a moot point--unless it came from the factory as a rifle, they won't be able to.)
The ATF's current interpretation of the law is correct under the Thompson Contender case. If a receiver is manufactured and sold as a rifle, it's a rifle and will always be a rifle unless you file a Form 1 to make it into an SBR. If a receiver is manufactured as a pistol, it can be turned into a rifle by adding a long barrel and buttstock, and then returned to pistol configuration--and thereafter you can swap back and forth as much as you like as long as it's never an accidental SBR while you're in the process of swapping components.
Now, if it comes from the factory as a stripped lower, you have two choices: make it into a rifle first, in which case it must always be a rifle, or make it into a pistol first, in which case it can then be made into a rifle, and then back and forth as outlined above.
Under federal law (state laws are a different story), there is zero requirement for engraving a receiver with words like "rifle," "pistol," or "SBR" in order to identify how it must be configured. In fact, even if a lower receiver is marked "pistol," it can be made into a rifle. The markings don't matter.
Now, to make an SBR, you can start with any type of receiver, no matter how it's configured. Fill out the Form 1, pay the $200 tax, wait 6+ months, and then you can configure as an SBR. Either the lower receiver or the barrel must be engraved with your name, city and state. People usually engrave the lower on an AR15. You do NOT have the option of creating a new serial number if the lower already has one--the ATF insists that you use the current serial number unless you're manufacturing an SBR from a hunk of aluminum, in which case there isn't one and you can make one up.
Once an AR15 lower is registered as an SBR, you can put any length barrel on it that you want and use any caliber. The ATF wants to be told if you make a permanent change from the information you listed on the Form 1, but if you have the "original" upper as listed on the Form 1, you can have any number of additional uppers in different lengths and calibers. Only notify the ATF if you get rid of that upper.
Slightly complicating matters as well is that your lower IS NOT an SBR when it's not actually configured as one. So if you remove the buttstock or put on a 16"+ barrel, it stops being an SBR temporarily. At that point you can cross state lines without notifying the ATF, or you can sell it as a Title I (non-NFA) firearm. The ATF would like you to notify them if you sell a registered receiver in Title I configuration so that they can remove it from the registry, but that's not legally required.
Oh, and you can use any buffer tube for a pistol build. Can be a "pistol" buffer, carbine buffer or rifle buffer tube. Doesn't matter. However, if you have a stock that can be attached to that tube, you run the risk of being prosecuted for "constructive possession" of an unregistered short-barreled rifle if the ATF catches you with both parts in close proximity.
I think that covers my understanding of the law. Dogtown tom, Sam1911 or RyanM, feel free to correct me if I'm wrong.
Aaron