Prowse did know, (or guessed) and was quoted in a small local newspaper article during a publicity event in 1978. That a larger publication didn’t pick up on it is a mystery.
http://io9.gizmodo.com/5684916/in-1978-darth-vaders-actor-spoiled-the-empire-strikes-backs-ending-to-a-local-newspaper/The version of Vader’s injuries that made the rounds when I was a kid was that Vader fell into a volcanic vent during a duel with Obi Wan. I think that comes from a tidbit Lucas dropped in an early print interview.
Pretty impressive that it went viral in the analog pre-internet era. Usually, it was just lurid urban myths like the one about the kid from the Life Cereal commercial exploding from mixing Pop Rocks and soda pop. 😉
Oh, many things went viral in those days. I just took months to propagate instead of hours.
I think Prowse either guessed correctly or had a hint from his character’s name.