A bit more information...
This mask has the filled in undercut on the tubes (ESB originals don't) however the filler doesn't match that of any other mask I've looked at so far.
The casting itself is thick, unlike the originals which tended to be thin. An example of this showing the FM ESB versus the hero ESB around the lower chin vent is below. The FM is just too thick to be an original.
Here's the addition to the undercut in the rear I was talking about before...note that the strap slot is not cut like the originals, nor is it even cut the same way as an authentic Farmer ESB mask. So why would someone add more to the rear undercut if this was an original?
On the inside of the eyes on the sides of the nosebridge one can see additional material added.
However, this is not the same as the TM ESB which also has added reinforcement on the inside of the eyes. It is unlike anything I have seen before and is not on any original ANH/ESB/ROTJ. I have been looking at tour masks but also cannot find something like this. Note that because of this the lenses are seated further back into the eye than on originals.
Also this image shows that the front side edge of the nosebridge on Vader's left is straighter than it should be....there should be a gentle concave curve to it on the top part. I'm not sure why. Nose slots are good.
I've been looking at the side of the U-shaped part of the nose on Vader's left and so far it isn't like anything I've seen before in the detail there. I've compared it to authentic ANH and ESB castings along with the Paul Allen original stunt ESB in images and so far nothing. I was hoping to relate it to a particular tour mask but so far no luck.
Of course the mounting ring isn't original, nor are the screws that fix it to the mounting plate. Below is a comparison of a mounting screw from the auction mask (FM ESB) with a screw in a similar location on the original Paul Allen ESB stunt mask. There's no relationship.
I had thought it had a bit in common with the 20th C but it is seeming less so since the 20th C doesn't have the kind of filled undercut on the tubes, along with these other differences I discuss above.
All I can say are the things that make it different from an original ESB at this point, and different as well from a confirmed Farmer ESB mask in some ways as well.
But then an obvious question is that given many of the more obvious differences like the mounting ring, why wasn't that detailed in the auction description as a description like that should have included any subsequent changes done to the mask outside of any claimed production origin.
Had Christie's consulted with me on this they wouldn't be in this position.