It's been 70 years since the letters of John Pryor were understood in their full meaning. That's because as a British prisoner of war in Nazi Germany, Pryor's letters home to his family also included intricate codes that were recently deciphered for the first time since the 1940s.
Pryor's letters served their purpose in World War II, as Britain's MI9 agents decoded the messages hidden within them — requests for supplies, notes about German activities — before sending them along to Pryor's family in Cornwall.
"There were two types of information buried in these letters," Pryor's son, Stephen, tells Weekend Edition Saturday's Scott Simon. "There is military intelligence going back about munitions dumps, about submarines that have been sunk, and information requests for British Military Intelligence in London to send maps and German currency and German ID, to help them with their escape plans."
After the war, Pryor lived a long life; he died in 2010 at age 91. But he also forgot the intricate code he used to communicate after being taken prisoner at Dunkirk. The letters came under new scrutiny recently after Stephen Pryor, the chancellor of Plymouth University, mentioned them to a military intelligence expert at the school. That led them to team up with a historian and a mathematician. Eventually, they cracked the code.
As an example, Pryor reads a segment of a letter: "I am pleased that I've got the two letters telling me of my cousin's latest event; how happy he must undoubtedly be."
The passage contains coded information about a submarine, the HMS Undine, Pryor says. And the code is far from simple.
"You take the first letter of every word in groups of three," Pryor says. "And then it goes into a three-dimensional matrix, which you have to remember in order to decode and get the sequence of letters to produce the name of the vessel."
The letters passed through German censors, and then through the hands of British agents, before finally reaching John Pryor's family. In cases where POWs sent information in code, intelligence officials "informed the relatives that some letters would read a little strangely," Stephen Pryor says.
The prisoners also used subtle cues, such as including certain words or underlining their signature, to signal to British intelligence that a letter contained coded information.
Stephen Pryor says that after the war, his father mostly kept quiet about the code, and about his wartime experience.
"But I can see now that he was among tens of thousands of other young men who gave up their youth in captivity," he says. "He and his peers took incredible risks, and that has only made me admire him, and all the other men, for their resilience and ingenuity."
You can read a longer passage from one of John Pryor's letters in The Daily Mail, along with its decoded meaning.