Stock = rifle.
No stock or ability to mount one=pistol.
Once a rifle, always a rifle.
It's actually a bit more involved than that, but suffice to say that a short (less than 16") barrel on a stocked lower, without proper licensure, is not a good thing at all....but you can put as long or short of a barrel as you ish on a pistol lower.
We're going the extra mile and marking specific lowers as "pistol" lowers so that there is no question as to their origin and lineage.