Frame

A pistol frame or a fire control unit is the serialised part — the component the law considers the firearm. Everything else is hardware. On modular designs like the Sig P320 the serialised unit lifts out and drops into a different grip module, which is why one purchase can become a full-size or a compact pistol.

Because it is the firearm, a frame transfers through an FFL exactly like a complete handgun: it ships to your dealer, you complete a 4473, and you are subject to the handgun age minimum of 21 even though what you are collecting has no barrel or slide.