have seen a lot of filed off serial numbers, this isn't one of them
I've seen any number of older Colt and S&W revolvers with butt stamped serial numbers that have been removed without leaving any trace whatsoever.
The idea that someone with moderate skills can't remove a stamped number from a flat butt without leaving signs of removal doesn't hold water.
Since I was mostly a Colt gunsmith most of the obliterated numbers I saw were on the old Colt New Army & Navy military issue revolvers.
These had not only serial numbers but also US Army or Navy stamps stamped in fairly deep.
I've seen a number on which there was no sign at all the number was removed.
Further, I've seen US issue 1911 and 1911-A1 automatics from which the "US Property" stamp on the frame was removed so well it was undetectable.
Never underestimate the skills of people who want a serial number to disappear.
True, most removal jobs are crude and noticeable as having been removed, but not all.