The receiver and or frame of the firearm is the serialized component and therefore the component that requires transfer by a FFL if purchased new. They must be serialized by the manufacturer per BATF regulations prior to being released for sale.
Nothing other than NFA items are "registered" in CO. A NICS/CBI background is not a "registration".
Ancillary components such as barrels, stocks etc are plain over the counter/internet sales.