I can't help you with phrenorton, but I consulted a friend of mine who's a Sergeant in the Army and a World War II trivia buff - what he doesn't know about WWII isn't worth knowing. He has never heard of a phrenorton, but he did give me the following history of Kilroy:

It seems that during WWII, a worker in a New York munitions plant decided to let the soldiers in Europe know that people back in the States were thinking about them. He started writing "Kilroy was here" and drawing the phrenortons on boxes of ammunition that went out of the plant (I'm not quite sure how he figured that the phrase "Kilroy was here" would let soldiers know that people were thinking of them, but...). Soldiers thought it was kind of neat, and it become one of the fads of WWII.

Not a real exciting explanation, but real life (even during war) is not always exciting.