[khrbtxyz]

Is this something special you have set up on your Mac?

I keep getting screenshots when I hit command-shift-3. https://support.apple.com/en-us/102646

[khrbtxyz]

Found it in the readline manual

https://tiswww.cwru.edu/php/chet/readline/readline.html#inde...

insert-comment (M-#) Without a numeric argument, the value of the comment-begin variable is inserted at the beginning of the current line.