I think he may have been an American who worked on the docks on the West Coast in the 20s and joined the Industrial Workers of the World, from which he was recruited by the Soviets as a spy. He may have been sent to Australia to gather information on American maritime activity there during WWII. He could have blended in as a dock worker. After the war, he may have fallen out of favor with his spymasters; perhaps he expressed his intent to abandon spying and seek a more normal life.
But I don't think he was murdered. I think it's unlikely that his handlers would have poisoned him and left him to die on a public beach when they could have just killed him quickly and gotten rid of his body. I think he became despondent and committed suicide. But first he did everything he could to cover up his identity, probably because he didn't want to expose Jessica Thomson who was assisting his spying.
Or not.
Perhaps. I think the spy theory is unlikely and is more the imagination of book writers. But it is not unfounded. A mysterious death. A book with a code. A woman's phone number who denies any knowledge.
But one of the significant elements that brought about the spy theory is that Jessica/Jo/Jestyn told police she gave a copy of the Rubaiyat to Alf Boxall several years before. She met Boxall in a hotel bar that military men often frequented. It appears she met him twice and he was shipping put and she gave him the book with an inscription. For a time it was suspected that Boxall was the Somerton Man, but he was found alive and had the book. He was working in military intelligence at the time he met Jessica, but he denies that he told her about that.
Maybe Somerton Man and Jessica were spies. Maybe Boxall was as well, or at least Jessica tried to recruit him.
She met Boxall in 1945. US/Soviet relations began breaking down at that time, but I'm not sure how extensive the Soviet spy network was at that time. If Jessica was a spy and Boxall was also or she had tried to recruit him, why would she tell police about giving him the book when it would be very unlikely for the police to ever make that connection? Also, I don't think anyone has presented evidence that Jessica or Boxall were Communist sympathizers.
There are, of course, many other theories. Jessica was involved with a man named Thomson. She presented him as her husband even though he was married to another woman. He was in the process of getting a divorce and when that happened a couple years later they got married. Thomson was involved in illegal black market used car sales (common at the time due to government restrictions). There are also speculations that he was involved in illegal baccarat gambling.
I think my preferred theory is that Jessica had a secret relationship with Somerton Man and that her son, Robin, is his. He had no family or friends. He found out he had a child. But she didn't want to have anything to do with him because she was in love with Thomson. He became despondent and went to meet her to ask her to marry him, even though he knew she wouldn't, but he want that last meeting before he killed himself.
After she rejected him, he killed himself. Maybe took some poison. Maybe she allowed him to stay the night and he stuck his head in the oven. Having his dead body there would result in a lot of unwanted explaining to do. So she, probably along with a close friend, moved the body to the beach.
That would explain why she told the police about giving the Rubaiyat to Boxall. Maybe she gave the Rubaiyat to all her lovers. Giving the name of someone previous would provide a nice red herring.
The code in the book may not really be a code. When I looked at I came to the same conclusion of many others that it appears to be initials of words rather than a cipher. It is four lines, like the quatrains of the Rubaiyat. It could be that Somerton Man made up his own quatrain to add to the Rubaiyat and wrote down the initials so he cold remember it.
One of the principal researchers of this case is a man named Abbott, who oddly enough married Robin's daughter. It appears she wants to test if Somerton Man is her grandfather. I think Robin's DNA is also available.
Hopefully DNA tests will show whether or not Robin is Somerton Man's son.