The clue is that he was wearing a white shirt ie in his No 1 (best) uniform. If he was a Petty Officer his badge would be in gold wire. As this is in Red thread he is what is described as "a (junior) rating not dressed as a seaman" - a seaman wore a cap without a peak, a sailor's collar and bell bottoms. This applied to Artificers who were not yet Petty Officers ie Apprentices and Leading Artificers; and also to Sick Berth Attendants.