Here's how it is with shotguns (and part of the reason there's a blank for "other" on 4473s now.)
If a pump shotgun with a pistol grip that has *never* had a buttstock on it is sold, it is not a shotgun. A shotgun, per federal law, is "a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder". A PGO shotgun is not. However it is also manifestly not a handgun because it is not "designed to be held and fired by the use of a single hand". It is not a short-barreled shotgun because it was not "any weapon made from a shotgun (whether by alteration, modification or otherwise)" and it is not an AOW because "The term “any other weapon” means any weapon or device capable of being concealed on the person..."
Therefore they exist in a sort of limbo, in that they are obviously firearms by law, but don't fit any specific legal subcategory (like bare receivers that have not been made into a gun yet) hence, "other".
The THEORY (which has not been tested in court; I wouldn't be surprised if MI has submitted something to Tech Branch though) behind what is being done here is exploiting the same loophole. Is this a rifle? No, because it is not "designed or redesigned, made or remade, and intended to be fired from the shoulder". Is it a pistol? No, because with the second grip it is not "designed to be held and fired by the use of a single hand". Is it an Any Other Weapon? No, because being over the magic 26" number it is no longer "capable of being concealed on the person..."