Class 3 parts would be things like lightning links, DIAS's, etc.
Lightning links, DIASs, machine gun receivers, suppressors, etc, would be "Title II Registered Firearms" not "Class 3" parts.
Seeing as the DEALER is a "Class 03 SOT Federal Firearms License holder" maybe Class 3 parts are like arms and legs, a spleen, couple of kidneys, something at or more likely
under 32 teeth, some hair (not much, though) ... all harvested from a demilled
dealer.
Hey, not much of a joke ... but I laughed!
(Of course, to your point, a barrel, bolt, stock, etc, are all just chunks of metal or "parts." Not regulated at all.)