[copperbrick25]

I looked at multiple messages on that website and the pattern seems to be triple letters are for single letters and abbreviations. My theory is because if you get a letter wrong in a word and "possible" becomes "possifle" then it is still intelligible, but if "u five one four" means u-boat 514 and you get the U wrong, then it could end up as A514 and mean something airplane 514, so to be safe the letter u gets transmitted thrice.

[smorrebrod]

I think your theory is correct. Back when messages where not checksummed, the French Navy used to do exactly this and sent abbreviations three times in a row. I'm quite certain that the French Navy was not the only one to do this since our communications techniques were heavily influenced by foreign nations like the US.