I have always been of the understanding that one can remove an NFA item from the registry.I think you are mistaken. Seems like you can indeed remove an SBR from the registry.
If I no longer want a short-barreled rifle (SBR) and I remove the “brace,” do I need to contact the NFA to un-register my SBR with an attached “stabilizing brace”? | Bureau of Alcohol, Tobacco, Firearms and Explosives
It is not a requirement to remove your SBR from the National Firearms Registration and Transfer Record (NFRTR); however, ATF highly recommends you notify the Government Services Branch (GSB) of the National Firearms Act Division to remove the firearm from the NFRTR registry. All NFRTR updates...www.atf.gov
What fees?