He was guessing for sure. I am pretty sure he or Kershener said that Prowse was saying: "Obi Wan killed your father" in the Bespin scene and he didnt find out the truth until the premiere.

I thought the grudge with Lucas comes from the fact that Prowse was rather possessive of the Vader part. He wanted to do the voice and be the face under the mask and make appearances in the costume.

I read (I think it was from a Hamill interview) that he would prance around the set in full Vader gear, MH was annoyed with him for not giving credit to the guy doing the fencing (Bob Anderson?)...Apparently he and Christopher Reeve, who Prowse trained for Superman, also had some falling out.

One interesting anecdote Prowse told (assuming its true lol) was that on Jedi they didnt want him to do the bit with picking up the Emperor and so they had rigged all sorts of things for a stunt man to do the lifting that didnt work-and finally they relented and he just went over and picked up the Emperor in one hand with no problem.

I think Prowse is right that Vader was the most iconic part in SW.