(I hope this link works)
Anyway, it’s a helpful guide for writing telegrams from 1928.
It says, in addition to many other quaint things,
"If you do not intend to stipulate that marks of punctuation be transmitted, write your message without punctuation and read it carefully to make sure that it is not ambiguous. If it seems impossible to convey your meaning clearly without the use of punctuation, use may be made of the celebrated word “stop,” which is known the world over as the official telegraphic or cable word for “period.”
This word “stop” may have perplexed you the first time you encountered it in a message. Use of this word in telegraphic communications was greatly increased during the World War, when the Government employed it widely as a precaution against having messages garbled or misunderstood, as a result of the misplacement or emission of the tiny dot or period.
Officials felt that the vital orders of the Government must be definite and clear cut, and they therefore used not only the word “stop,” to indicate a period, but also adopted the practice of spelling out “comma,” “colon,” and “semi-colon.” The word “query” often was used to indicate a question mark. Of all these, however, “stop” has come into most widespread use, and vaudeville artists and columnists have employed it with humorous effect, certain that the public would understand the allusion in connection with telegrams. It is interesting to note, too, that although the word is obviously English it has come into general use In all languages that are used in telegraphing or cabling."