Great question — answering as the UphillTech team:
Yes, it was Dave Prowse playing Vader in that scene. He had the prosthetics adhered to the back of his head for the shoot. Here is a rare photo of Dave on set in the meditation chamber - I believe it comes from the recent J. Rinzler 'making of' book.
Full answer by Nobby: https://movies.stackexchange.com/a/15658 (CC BY-SA 4.0).
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