Thomas, I know this could be out of my element, and I haven't studied all the original casts, but I did find this to be a truly cool cast. I didn't realize you had it for almost 10 years.
Reading everything that you were saying about the cast, and a lot of the details that you pointed out I couldn't help but start to ponder its place in the scheme of things.
I started thinking about the black paint job and how many layers of paint, the use of an alternative resin, and the multiple modifications. The mask appears to be at one point equipped with the hardware for the two-piece front and rear set up and then that was abandoned and removed. Then you have the filled in holes at the top near the 3M tabs, the holes in the side that have been filled in, the ends of the tubes having been cut off, the foam inside as if it was actually meant to be worn. Also, the addition to the right side, and lastly the holes drilled in the grille areas.
I started to think about why would this helmet, have all these different things, things added and then removed. I think you're right about the holes being drilled so that someone could breathe. But why go to all the trouble to do such a serious paint job before any of the grills were cut out? And the only thing I can come up with, is a 'mule'. A test mule. a piece used to test all the various options, modifications or ways of doing things that they were considering.
If they did use this helmet try out the two-piece system (which would also explain the foam inside and the holes being drilled so someone could breathe) that would have to make this a fairly early preproduction ANH cast.
I suspect this might have been the piece they used to work out all the bugs. Even the green paint may have been a test.
Truly a fascinating piece.