Presumably there would be control codes that would be sent that surround the message.
For example, in my ham radio days I remember using “BT” (written as BT with a line over it) as a section separator, sent as “-…-”, the code for B and T squashed together.
(See Prosigns for Morse code for more detail.)
There were many of these codes for various purposes, and I imagine that a business that sends lots of telegrams would have additional “envelope” stuff surrounding the message.
The 1928 manual quote alludes to this: …unless attention is specifically called to punctuation marks by counting them in the “check,”
I’m assuming that the “check” refers to some kind of parity check or maybe a character count.
You may be interested in Final Salute by Jim Sheeler. He wrote a long article by that name for the now-defunct Rocky Mountain News on the casualty asisstance officers, following one officer, Major Beck, for an entire year. He and the photographer won a Pulitzer Prize for the column. Used to be avaialble on the net, but since the Rocky Mountain News went under, it doesn’t seem to be up any more. It was heart-breaking.
He’s now expanded it into a book. Looks like it’s available as an audio-book from NPR: