Not really, but functionally correct.

A lower without a buttstock is not a rifle or a shotgun, so does not fall under the "18 or over" exception for rifles and shotguns. As such, it can only be transferred to someone 21+ by a licensed dealer. A lower with a buttstock attached (so, basically, complete lowers as opposed to stripped) is considered a long gun, and still falls under the 18+ exception for rifles and shotguns.

There's a checkbox on the new 4473 for "receiver" in addition to "long gun" or "hand gun".