The thing is I am only talking about federal law. Every state has their own little quirks and there is no way with my feeble little mind I could keep all of them straight even if I wanted to. Federally there is no need to have a lower "papered" as a pistol by the maker; that is pure BS that the makers use to try and scare you into paying more money for their "special" lowers.
Now, some states MIGHT have a law stating that the lower must be papered from the maker as a pistol lower. I know some states require that for a lower that is going to be used in a pistol build the buyer needs to jump through the pistol buying hoops that are special to that state. Of those types of regulations I will not even try to address.
It is always prudent for a person contemplating a pistol build such as this to check out their state laws. IF the state says that a pistol has to be papered by the maker as a pistol then I guess you need to pay the extra money for a special pistol lower, but most states do not require such nonsense.
The only really big issue on the federal level is what if a person 18 years old wants to buy a lower for a pistol build. Since federally a person needs to be 21 to buy a pistol from a dealer, then it would make since that a person doing a pistol build needs to be 21 to buy the lower also, and not just 18 as is required for a person buying a rifle or rifle lower. However, that is a topic of discussion on another day.