AR loser receivers - pistol v. rifle

Trying to find chapter and verse in the ATF regs about whether or not a bare AR lower receiver originally reported to the ATF by the manufacturer as a rifle can be built into a pistol if it has never been assembled into a rifle prior to the pistol build. So far not much joy.

Best I can come up with is this 2011 ruling that had to do more with combo "kits" that allowed switching Thompson Contender receivers from rifle to pistol and back at will. https://www.atf.gov/firearms/docs/r...red-rifles-rifles-configured-pistols/download

My instinct is once a rifle always a rifle (or properly registered SBR), and once a pistol always a pistol, regardless of whether or not the subject lower receiver has even been populated with parts. But IANAL. Can any of you regulation gurus point me in the right direction? Have a good repeat customer who is a LEO and who wants to build a personal AR pistol on a new rifle lower receiver that has been kicking around in his gun safe for several years. I would like to warn him off if he is about to step into a bear trap.

By the way the heading is supposed to be "AR lower receivers. . . But if he builds a ticket to Club Fed then I guess "loser" receiver applies.