Well it was No.6 who found the body when he woke up on the beach that morning, but I don't think he was the one who did it. Sure No.6's first thought was to go through the dead man's pockets, otherwise he wouldn't have found the wallet, or the radio from which he might hear word from the outside world.
Later No.6 would use the cadaver for his own ends, to use it and send the outside world a message, to whoever may find it, that he was actually alive. No.2 would then have the body amended slightly so that it is No.6 who has died in an accident at sea! Very important this dead man has suddenly become, even if he wasn't very important alive.
But who was he? A citizen of the Village of that there can be no doubt, the picture of the dead man together with a young woman, sitting together by the fountain pool in the Piazza proves that. But how had he acquired the radio, and for what purpose?
The radio message received by No.6 was obviously intended for the dead man, so could he have been a plant in the Village? But that would still not account for the radio, unless there were two such plants in the Village, the one in a position to gain the radio for the dead man. You will recall how No.12 in Administration aided No.6 that time, in getting two security pass discs and "Top-hat" uniform for No.6 in ‘The General.’
Of course we may never know just who the dead man was. All we do know is, that he was a citizen of the illage, that he knew a young woman there, so it might be a good idea to track her down might be a good idea. That he had a radio cannot be denied, nor the fact that he was out and about either very early that the morning, or like No.6, he defied the curfew and spent the night on the beach. How did the man die? Well I think he died a horrible death of suffocation, by the Village Guardian, past curfew time or not.