I've compared the SL mask to the original ANH mask, and that rear edge in terms of how it is shaped on the SL is identical in every way with the screen mask.
So then how could the TD have more material going back further, a rounded rear edge in that area (not straight viewed from the side), and a complete indentation? In fact if you look at the SL and TD side by side, it is pretty obvious that the additional undercut where it curves out was cut flat and that led to the straighter shape we see on the SL, and also on the screen mask. Not only that, but just at the right corner of the rear of the SL mask has what looks like some reworking to make it more of a corner and less of a curve, the curve left over from the time it had more material back there (the curve that matches the TD in that area).
So what does this mean? There are two possibilities. One is that the TD ANH did not come from a mold taken off the original ANH mask, because if the original mask is missing that extra rear section, then how could it be on the TD? But the grill in the mouth of the TD refutes that possibility.
The other possibility therefore is that the TD is cast from a mold taken from the original ANH mask, but before that area was trimmed on the mask, and therefore the mold for the TD (not necessarily the TD itself) would have come from the original ANH mask before it was trimmed in the rear. Now, the trimmed version of the original ANH mask has the original paint on it, so it would have to have been trimmed during the ANH production.
Along with the other early details I've shown, this is really the proof I had been looking for that the TD is from the ANH mask at an early stage of its life...
Answering my own posts again... :confused
Anyway here's what I meant....overlay of the rear right bottom corner on the TD and SL. The 60/40 overlay on the left shows the amount of additional undercut the TD has in that area. Also shown is the curved "corner" indicated by the black frame, and how it looks like there was some work done there on the SL ANH corner to go from being rounder to being more square, and the boundary of the round part matches the rounded edge of the TD. Since the SL matches the screen mask perfectly in terms of that rear edge and corner, it is strange that the TD has more, but also that the original is trimmed just where one would take off that extra material on the TD to make it a straight edge, and the corner changed to match that straight edge.

And just a reminder that the front/rear mold seam for the full head mold that the original ANH mask came out of would be well ahead of this area, so there could have been initially more undercut that was later trimmed back to be straight....and the TD could be showing how the original mask looked before it was trimmed during the ANH production.