mh-letter-mode-help-messages

mh-letter-mode-help-messages is a variable defined in `mh-letter.el'.
Its value is

((nil "Send letter: \\[mh-send-letter]    " "Open line:        \\[mh-open-line]\n" "Kill letter: \\[mh-fully-kill-draft]    " "Check recipients: \\[mh-check-whom]\n\n" "Insert:\n" "  Current message:      \\[mh-yank-cur-msg]\n" "  Attachment:           \\[mh-compose-insertion]\n" "  Message to forward:   \\[mh-compose-forward]\n" "  Signature:            \\[mh-insert-signature]\n\n" "Security:\n" "  Encrypt message:      \\[mh-mml-secure-message-encrypt]\n" "  Sign message:         \\[mh-mml-secure-message-sign]\n" "  Sign+Encrypt message: \\[mh-mml-secure-message-signencrypt]"))

Documentation:
Key binding cheat sheet.

This is an associative array which is used to show the most
common commands. The key is a prefix char. The value is one or
more strings which are concatenated together and displayed in the
minibuffer if ? is pressed after the prefix character. The
special key nil is used to display the non-prefixed commands.

The substitutions described in `substitute-command-keys' are
performed as well.