Mark Twain? F. Scott Fitzgerald? Anonymous?

Question for Quote Investigator: Would you please explore the provenance of a piece of writing advice that I’ve seen several times. Here are two versions:
One should never use exclamation points in writing. It is like laughing at your own joke.
Cut out all these exclamation points. An exclamation point is like laughing at your own joke.
The first remark has been attributed to Mark Twain and the second to F. Scott Fitzgerald. Sometimes the term “exclamation marks” is used instead of “exclamation points”. What do you think?
Reply from Quote Investigator: Mark Twain did mention exclamation points in his essay titled “How to Tell a Story”, but the context only partially matched the statements above. Details are given further below.
In the final years of F. Scott Fitzgerald’s short life he began a love affair with Hollywood journalist Sheilah Graham. He acted as her mentor in the domain of literature according to Graham’s 1958 memoir “Beloved Infidel”. She wrote a five-minute script for her inaugural radio program, and she showed it to Fitzgerald who spoke the words of advice under investigation. Boldface has been added to excerpts:1
“You don’t mind if I reword it here and there?” he asked. And though tired from his own writing at the studio, he sat down with a stubby pencil and a pack of cigarettes and painstakingly—and completely—rewrote my copy. He worked with the utmost concentration and as he worked he twisted the hair above his forehead so that a tuft stood up, as on a kewpie doll. It gave him a strangely boyish appearance. “Cut out all these exclamation points,” he said. “An exclamation point is like laughing at your own joke.” He underlined words I should emphasize, corrected my grammar.
Here are additional selected citations in chronological order.
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