Just a clue, the ANH were first so are made of PP, HDPE or something similar and painted White. All the details, like the traps, tears, frown and vocoders were hand painted. ESB were rehashed ANH helmets and some new ABS and had decals in the tears and traps and had black pointed frowns. Both ANH and ESB had green flat lenses apart from the ABS closeup hero helmets that had bubble lenses.
ROTJ are Recast helmets and are more squashed, warped and considered the ugly cousin. Although I have grown to love them as just different. They nearly all had bubble lenses although some had flat green. The decals were screen printed vinyl and the frowns were black and had either square or rounded ends. Made of pinsea/haircelll ABS. Inside they had helmet liners wereas ANH and ESB had star shaped foam.
The ears were screwed on with 3 brass screws on the stunt helmets on ANH and ESB. The Heros had 2 screws.
In ROTJ they were glued on.
There are other differences but those are the obvious.
Hope this helps.