When you buy an SBR, you use the manufacturers markings to fill out the ATF Form 4 (Application for Tax Paid Transfer and Registration of a Firearm) - afterall, the actual manufacturer is the "Manufacturer," in this case. You can't touch it until you get the tax stamp back - it sits in your dealer's safe.
When you build your own SBR, YOU are the "Manufacturer," and you have to fill out the ATF Form 1 (Application to Make and Register a Firearm). You need to send the lower receiver (the "registered" part of the firearm) off for engraving with YOUR information on it, as you are the manufacturer, and that info needs to be listed on the Form 1. While you're waiting for the tax stamp to come back, you can legally have this firearm in a pistol configuration, and shoot it. When the tax stamp comes back approved, in your hands, you can then convert that pistol into a Short Barreled Rifle.