It really depends upon how the lower is initially transfered on the first 4473. Ask your processing dealer.

Some manufacturers do not designate rifle / pistol, and some even mark their receivers "Multi Caliber" (Tactical Innovations, for instance). Spike's on the otherhand designates them with caliber and pistol/rifle.

When we sold stripped receivers we completed the 4473 as "Stripped Receiver" -- but only on brand new, virgin, never assembled receivers.

Once it is recorded on the 4473, what it is is what it is. If you assemble a pistol off of an existing rifle receiver you've just built an SBR.

If someone is quoting you $200 more for a pistol receiver over a rifle, I'd get the tax stamp and register it SBR. Then you can configure it any way you wish (except machinegun), and never worry about how you put it together the night before the shoot.