> Separately, I am always a little confused by the idea that you cannot "trace" these firearms.

It's presumably a misunderstanding of how investigations work. They're paperwork people so the assumption is that the serial number is of vital importance because it's what's on the paperwork, and if something could exist with no serial number then the entire system is in danger.

Meanwhile the serial number is overall not even that helpful. If you catch the suspect with the weapon in their possession then it doesn't matter that much what the serial number is, what matters is if the weapon they had matches the forensics. By contrast, if you don't recover the weapon then you don't have the serial number anyway.

The only case where a serial number would really do anything is if you recover the weapon after the perpetrator already tried to dispose of it and want to try to use the serial number to identify the original owner. But in that case the perpetrator can leave you without a serial number regardless by just filing it off. It doesn't really buy them anything for it to have never had one to begin with.

Serial numbers are security theater on the same level as TSA checkpoints at the airport.