TM is right, no square frowns show up in ROTJ, except that my square frown helmet came from the set of ROTJ. It may not have been used and as it was given to a young boy (whose family had connections in the business). It could have been a spare backup helmet or deep background, but there is zero evidence it was used in the film. The visit was documented in a Star Wars Insider article.
But TE's helmet is detailed like a screen used helmet but had the ANH style ears.
DM Wearing the Screen style finished helmet with the ANH/ESB ears
The whiter helmet at the front is from the 911 Charity auction and has a LFL COA with it, it is the same shape as my helmet, whereas the TE original and replica are much fatter in the face with the ANH/ESB ears. Weird...
I don't think we can be sure of anything with any certainty, the mixed and matched all sorts.
Joe