When you say "knowledge of how to use", do you mean the average person understood that each letter was encoded into short or long beeps?

Or are you asking if the average person had the ability to translate a piece of written text into and out of morse code?

In the book "Between Silk and Cyanide: A Codemaker's War" by Leo Marks, the author relates that the people receiving Morse code messages would recognize the senders by their 'hand.' They got so good at it (lots of messages) that if someone was sending a message with all the right code words, but the pattern was slightly off, they would flag it as possibly compromised. That is, the spy was caught or dead and the enemy was trying to pass themselves off as the spy. An excellent read.

As a follow-on question, if anyone has answers or other sources- how frequent was the use of written Morse code? Outside of telegrams, radio signals, and other long distance-communication, was written Morse code really utilized for anything?

I ask because my great grandmother used to write me notes in (what she called) shorthand. After some research, I realized it wasn't any of the standardized shorthands, it was Morse code. She was a secretary and not, to my knowledge, involved in anything that would've required its use.

In Andrew Carnegie's biography by David Nasaw, it was noted how Carnegie learning Morse code at a young age was very special at the time. That was around 1850 in the early days of telegraphy though.

In Reminiscences of a Stock Operator published in 1922, alledgely a biography of Jesse Livermore, there's a story about a pair of brothers learning Morse code after a telegraph operator made a mistake on their message.

Both of these stories would seem to indicate every day people would not have knowledge of Morse code. And there's really no reason for the average people to know it. Post offices transcribe the code before delivering the message to its recipient.