First, understand how the FRT numbers work...
The FRT number is essentially comprised of 2 parts... the FRT # and the "Child" numbers.
Example... FRT #12537 is the main FRT # for Rock River Arms LAR-15 "Pistol" AR's. But then there is the "-#" portion which is the "child" entry.
A Rock River Arms LAR-15 "Pistol" with 10.5" barrel, in calibre 9mm and with 10 round magazine has a child entry of 7 so the FRT #12537-7 denotes a Rock River Arms in that specific configuration. #12537-15 is a .223 caliber, 7" barrel, 9 round magazine version of the same gun. And so on for as many "child numbers" as are necessary to describe each different variation (caliber, barrel length, mag capacity). Some FRT #'s can have 100 different child entries.
When a dealer or importer wants to register the lower as a "Receiver Only" without a barrel length, mag capacity or caliber then he simply registers the firearm using the main FRT # without specifying any child entry and he ticks off the spot on the form that states "Frame Only".
This basically applies to any firearm... just drop the child entry number and specify "Frame Only".
Mark