Yeah, I refined the LFS one a bit as I remeasured everything. Of the two top protrusions, the top one was a little thicker and the lower one came out a little thinner, but there seems to be a clear difference between the jaw pistons.
M1 left - newer LFS with the most extreme version of the lower top ring in the middle - original LFS model right.
I've only found the LFS version on some endoskeletons, though I don't really have that much reference showing them in great detail so more than I've found can have them. But the majority has the thinner version, some as thin as the M1 kit, others about half between the M1 and the original LFS version. So I guess both are valid. I'll also make the 3rd version, where the lower ring is a little thinner than LFS but not as thin as the M1 and then people can just decide on their own which to use.
Thinner lower ring:
T1:

T2:
T3:
IconsofSciFi - EMPmuseum:
Tokyo Japan Museum Robot Science
01:

02:
Tomenosuke Shoten shop - Japan:
Thicker lower ring:
PropStore - 2018-09-20 - Entertainment Memorabilia Live Auction - Lot 545 - T2 - Terminator Endoskeleton:
Terminator 2 3D Attraction Store Window - Universal Studios Hollywood - though this one has a weirdly warped skull:
