I'd wait around for a second opinion, but my understanding is that if it's never been assembled into anything at all, then you can use it for a pistol build.
At the time the stripped lower was originally sold, it should have been recorded as an "other" on the 4473, which would allow it to be built into either a pistol or rifle configuration.
But if the lower originally came attached to a rifle stock, then it's a rifle lower, whether or not the lower was ever built up.