There is a lot of misinformation in this thread.
1. It’s possible for anybody to see if it were FA-10ed as a rifle if it were part of the “accidental” leak that the firearm records bureau did a couple years ago.
But that doesn’t matter … because…
2. FA-10ing it as a rifle means absolutely nothing in terms of legality of turning it into a pistol. There are plenty of people who FA-10ed lowers without building them into anything. And the state FA-10 form has zero relevance to the federal consideration of turning a rifle into a pistol.
Do you know if the previous owner actually turned it into a pistol? More importantly, are you sure the prior owner didn’t briefly turn it into a pistol before turning it into a rifle?
I ask because, for every single one of my rifles that I bought as lowers, I threw a pistol upper on them with no stock first. Therefore every single one was a pistol first and I can go back and forth between rifle and pistol as much as I want.
If it was purchased from the factory as a rifle, you can’t do that.