So i did this little diagram.. this pic is from the costumes book. it is the ESB hero (left)and the PP3(right). I still believe that the ESB helmet is unique compared to the other helmets.
The green line is vertical (or very close to it. I duplicated the layer and moved the line with the arrow key in photoshop as to not skew the line at all) i followed the line angle of the left ear on both helmets with the yellow lines. You can clearly see the angle difference there, as slight as it is, but it IS there.
The orange line is the different angle of the mandible on the one side. Again you can see how the PP3 is slightly angled (maybe this is partly where the MR pinch came from?) and the ESB hero has it more forced outward toward the bottom.
The light blue line is the difference is the bottom width of the visor. The line is the full width of the visor on the PP3. Again i duplicated the layer to keep the line the same size and just moved it over with the arroe key to put it at exactly the same place as on the PP3. It comes up just short on the ESB. Thus showing that the visor was likely pushing out the mandible which in turn has pushed the side of the helmet outward slightly creating more flare on the one helmet. It also made the "squint" on the visor area on that side less, or pushed upward on the ESB than the PP3. It would make sense that it may have also pulled down the brow, assuming it wasn't fully cured when the visor was installed. I know from experience that you can manipulate a Fiberglass cast to some extent if its not fully cured. The not fully cured fiberglass theory was also mentioned in Ponte's paint thread as to why there is the bulge on the upper cheek on one side of the ESB helmet only. The epoxy from the mounting plate likely warped the fiberglass, before it was cured. Again leading me to believe the helmet was pulled from the mold and assembled before it was fully cured, which can lead to plenty of distortions on a cast. Most of the anomalies seem to be on the left side of the helmet (right as we look at the pics) It looks like the entire cheek panel is twisted.
All these things are very slight but i think are something that can be seen, even if someone cant pinpoint it, the ESB helmet does have differences from the PP2, PP3 and ROTJ helmets.
I think the PP2 and PP3 are basically the same, meaning they were obviously from the same mold and neither suffered any odd damage or warpage. The MR and MF/CTM/MDF were derived from these same helmets and i think thats why they all look similar in size and shape, excluding the size of the MR.
I'd also like to add that believe that its pretty safe to say ALL the helmets came from the same mold. I think the ESB was handled differently when it was cast, pulled etc and thats why it has the anomalies it has.