They often started with 'My Dear Mummy & Daddy' and talked about mundane things such as gardening and a 'vegetable patch'.
But the British wartime letters actually contained hidden messages, which have only now been deciphered by academics.
It is also the first time the captured serviceman's family has been able to understand the secret messages.
Sub Lt Pryor was captured at Dunkirk in 1940 and sent to a prisoner of war camp.
He was held for the next five years but as a reward for good behaviour, he was allowed to send letters home to his parents in Saltash, Cornwall.
Those letters contained secret messages for the British military - and involved a fiendishly complex alphanumeric code.
In one letter, he hid intelligence about the sinking of a British submarine by writing about the PoW camp's vegetable garden.
He wrote: 'Many seeds are left, being saved from several plants which did very well some time ago.
'Our last year's harvest was extremely good. Well worth repeating again for this year.'
But it meant: 'HMS Undine attack failure. Trawler depth-charged, scuttled in 70 feet, three burnt.'
From this, experts were able to deduce a coded message about the sinking of the British submarine, the HMS Undine, which was destroyed in 1940.
Scroll down for complete breakdown of how the code was cracked.
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