Well, normally if it's an Emergency stop button, the collar is to prevent some jerkass from turn it back on and potentially killing someone that's half-inside the machine trying to fix it. (although there's supposed to be lock-out/tag-out for that as well, but...)
And buttons with rotating collars normally have markings as such as well, but I could understand if the markings had worn off it over the years.
They are also much more expensive than normal mushroom buttons, so it's not a cost thing.
Biting the hand that feeds IT
