The first draft of Mind of Evil was written before the Master came along, so the original villain got rewritten into the Master. In UK prisons some prisoners do get to wear civilian clothes. Not lifers like Mailer, but the clothes are accurate for the time (compare with the sitcom Porridge). Some hasty rewriting was needed when the Army troops guarding the real missile they'd loaned (yep, it's real. Not a prop) turned up in uniform. So the Master and Yates discuss how they're mercenaries in fake uniforms.