A little late - though as the subject has come up elsewhere…
it’s Prowse, I think this picture might only be in the e-book of the making of.
Our good friend Jambe Davdar says it is from the e-book version of Rinzler’s ‘Making Of ESB’ book too - and that it is indeed Prowse…
https://twitter.com/jamieswb/status/961615108475703302
‘Rarely seen The Empire Strikes Back video showing Dave Prowse in the famous “under the helmet” prosthetic.’
(keep watching until the end of the 39-second video)