I think this seam line occurs on other skulls too. For example, I think if you look at the Tomenosuke and Lightstorm skulls, you can barely make out a bit of wonkiness and cleanup. There are hints of offset, and I actually think that is the reason skulls are often missing 1 or 2 screws around the rear port. It's because they ran a mold seam across the screw area. And I bet that as they cleaned up these skulls they had to use filler and sanding which wiped screws at that seam area if they weren't careful or the offset was bad enough to cause them to use more filler. Check out the collage and you'll see what I mean.
I haven't gone back to closely examine all T1/T2 skulls for hints of that particular seam, but I wouldn't be surprised if found it on other skulls.
-Dana