(Trying to paste in some of the previous pics for visual examples)
The most common stunt helmets are the kind you see at the Lemonade stand.
Those were made specifically for the parachute stunts. They made vacuum-formed plastic shells in usually about 3 or 4 pieces and then assembled them around an actual sky-diving helmet. They were intended to be "break-away" so that the stunt guy could grab the vacuum-formed shell and literally just break it off of the real helmet if or when he needed to. They THOUGHT they were going to go through a ton of them so they made quite a few of these. They are easy to spot because they are always splitting along the side seams like the one at the Lemonade stand.
You can also recognize the parachute helmets at a distance because the eye lens area has been cut much larger.
The stunt in the picture I posted below is unique as far as I know in that it wasn't a "break-away" helmet (it's the middle top). The helmet on the right is a screen-used hero currently residing in it's new owner's private collection and of course the one on the left is the original prototype. I can't give out details of this stunt due to it currently residing in another private collection but I did get the okay to post this particular pic which includes it.
There has been tons of speculation as to other so-called "stunt" or "production" helmets but very, very little of it is confirmed. According to the propmaster that worked on the movie, ALL of the helmets were identical but that's not entirely true. All of the HERO helmets were identical in most details. There clearly were the vac-formed parachute helmets and the stunt in the picture I posted which are all confirmed as "original" but beyond that, I view any talk of resin "stunt" helmets very skeptically without serious evidence they were even production made.
Part of the problem is this... The original hero helmets were made for the movie production by one prop shop (after it was sculpted by Kent Melton of course who was unaffiliated with them). I'm not even 100% sure if that same prop shop had a hand in the making of the parachute helmets at all or any other "stunt" type helmet. Those guys also made extra castings for themselves and for trade fodder or whatever. (I'd have done the same thing I imagine - that was before most people had even heard of collecting "props".) But then Disney came and took all of the molds and made more helmets for a whole variety of uses... park characters, publicity, marketing, ad nauseam. They primarily made helmets of fiberglass - not resin - and they weren't finished with the detail and care that the original prop shop took for the movie helmets.
So, between non-production copies made by Disney and non-production copies made by the original prop shop, some finished, some raw, getting out into the world of collectors, untold numbers of them have been passed off as "original production" copies and "stunt" copies which make figuring out what was a truly legitimate stunt or production copy nearly impossible.
There are a few exceptions like if you got a copy directly from a guy that worked at the prop shop like ER did. Those instances are very rare.
The Acme helmet is theoretically a casting of an "original" but in all likelihood it's at least 2 generations removed from a screen-used original casting but more likely 3 or possibly even more. I do have pics of the hero next to an Acme casting that I'll see if I can dig up for comparison. It's quite accurate despite any generational issues with the notable exceptions of what I pointed out in that other thread showing the Acme helmet I finished.
This is also what happens when a prop gets into the wild and starts breeding unchecked. You can't rely on anybody's word 100% because even the guys that worked on the movie sometimes remember things from only a certain perspective and forget some other detail. That's also why I put so much weight in corroborating information.
Out of my decade plus of completely obsessing over Rocketeer helmets, nine times out of ten, if somebody says they have an "original," "stunt," or "production" casting of a Rocketeer helmet they either don't know what they're talking about or are lying about it. Usually it's innocent ignorance but ignorance none-the-less.
I learned the hard way that in screen-used/original/production collecting, authentication and provenance are king.
(NOTE: I'm adding some further info to this post so you might see the edited time change.)