According to this thread - https://originaltrilogy.com/topic/Actor-of-Darth-Vaders-bald-head-in-ESB/id/16462
it is Prowse; unfortunately the picture is apparently only in the e-book version of Rinzler’s Making Of ESB book (I’ve had a look through the physical book version and can’t find it).
Our good friend Jambe Davdar says it is from e-book version too - and that it is Prowse…
https://twitter.com/jamieswb/status/961615108475703302
‘Rarely seen The Empire Strikes Back video showing Dave Prowse in the famous “under the helmet” prosthetic.’