Clients often intend to file a Form 1 and manufacture a Short Barreled Rifle from what is a pistol. Once the Form 1 is approved, the manufacturing bit is easy. Just add a stock.
However, what is more difficult is figuring out what to engrave and where. The what, name of manufacturer and location, is required by the BATFE. (And they are even so strict as to require a set depth for engraving!) The where however is firearm specific and a bit trickier.
The manufacturer, that’s you or your trust, must engrave the firearm, which in the eyes of the BATFE is the frame or receiver. For an AR-15 this is the lower receiver. Easy, and it allows you to swap out uppers.
For a Broomhandle Mauser this is the pistol itself. (Note: Don’t go engraving a historic firearm without doing your research. There are exceptions for curios and relics that qualify.)
The difficulty appears when you use a modular stock meant to fit on many firearms. For example the Sig Sauer Adaptive Carbine Platform (ACP). On first pass, you will want to engrave the ACP itself. But this is the stock, and the BATFE will not accept this! Instead you must engrave the frame or receiver of the firearm you actually intend to use in the ACP. (And that pistol will still be an SBR in the eyes of law even without the ACP.)
In summary, research before you manufacture to ensure you engrave the frame or receiver. When in doubt, or if you just need a second opinion, reach out to a professional firearms attorney. I have helped many South Carolinians with a Form 1, and I would be happy to help you determine your options and which option will give you the greatest flexibility.