Excellent article, Peter!
Learning code was more of a "thing" in ground school in the 1960s when I learned to fly. I still remember most of it, though for the life of me I can never remember 'F', 'G', 'J', 'W', 'Z' or the numbers.
This reminds me of the journal written by my wife's uncle, who joined the Navy in 1940. It read in part,
Upon returning to Pearl, I was transferred back to the radio shack. I taught myself how to read code, finally got up to twenty-five words per minute, which meant I could go on the skeds, which are the messages coming in from radio stations. We copied Radio Honolulu, Radio San Francisco and Radio Washington (all US Navy stations), which was the method used to relay the word to all ships and stations.
[...]
At 3:32 AM "LZT" December 8th, Manila time, the message came across on the sked channel, which was manned by Taylor, 3rd Class Radioman. I was the supervisor. Taylor was having a hard time copying the plain language, so I attempted to find a place to plug in my earphones, so I could help him. Both of us were quite nervous, but copied it down. The message was Pearl Harbor and Army Air Force locations are being attacked by the Japanese. Govern yourselves accordingly and put War Plans Nr 367 into effect. I flew down to the Captain's cabin, then the Exec's, then Officer of the Day.