Lowers with the "pistol" marking are a scam. There is no legal requirement for the AR pistol to be marked "pistol". As long as the lower has never been configured as a rifle it can be made as a pistol. If initially built as a pistol, it can be later configured as a rifle then back to pistol.They're the exact same thing with a different stamp on them; they should be the same price, pistol lower MAYBE being five or ten bucks more.
When filling out the paperwork there are 3 options, Handgun, long gun(rifle,shotgun), and other firearm(Frame,receiver,etc.). Make sure who ever does the transfer checks "other". You can't legally change a rifle from a manufacturer to a pistol or pistol into a rifle. It's all how the original firearm is registered.Is there any document you need to fill out that says it has never been built as a rifle or is it your word against whoever asks?