Yes I do but it's going to take a lot more than some girly batting her eyes at me to get it.
Damn I hate being in the position of even appearing to agree with the NYT but…
A serial number is just a number. Unless it is recorded somewhere, in connection with some event like transfer of ownership it remains just a number. So a rifle is untraceable if the serial number has never been recorded. Not all states require this, and it is my understanding that until fairly recently few did for long guns.
It is my understanding that the rifle is supposed to belong to Robinson's Grandfather (possibly as a war trophy?) in which case it has probably never been seen by a bureaucrat, and so has no official record of ownership and hence would be "untraceable".