I wonder if Obi-Wan ever sees Anakin again after the duel? It is possible that he has never seen Darth Vader in his armored suit and helmet before their duel in ANH. In fact, Obi-Wan almost looks a little surprised when he sees him. Perhaps he felt the presence of Anakin before they met in the hallway on the Death Star, and was shocked to see what he looks like. I don't know whether people in the Empire have news broadcasts and photos of people like Darth Vader. Maybe Obi-Wan never saw the black clad Sith Lord until the fight to his death. He seems pretty isolated in his little Tatooine hut, it is possible that the meeting with Darth Vader on ANH was an unpleasant surprise to Kenobi.