T1 was a small production, so from what I have gathered there was 1 complete endoskeleton + a half body one (could be the 1 that is just taken apart and used accordingly,) and then some extras for various bits for various effects. We know they made more than 1 skull, because the Propstore auctioned T1 blow up skull exists.
For T2 there were 5 complete endoskeletons, though, JamesCameronOnline states only 4 was made - but there are 5 in the picture. Is one of them the T1 with some parts replaced with T2 parts, such as the hands and knee? Who knows. That site also states they refined things on the endoskeletons, so they could have refined the skull too. But besides the rear cover being different it seems to be pretty close to T1.
The EMP Museum one is a screen used skull, so one of the production skulls. All the ones I showed except for the Gift Skull are production style skulls, so the closest you can get. The Gift Skulls are probably the closest you can get after that. But they have M1 temple inserts and various grooves and details changed from the production style look.
At least that's just my take on things.