Variable: menu-prompt-more-char
menu-prompt-more-char is a variable defined in keyboard.c.
Value
32
Documentation
Character to see next line of menu prompt.
Type this character while in a menu prompt to rotate around the lines of it.
Source Code
// Defined in /usr/src/emacs/src/keyboard.c
DEFVAR_LISP ("menu-prompt-more-char", menu_prompt_more_char,
doc: /* Character to see next line of menu prompt.
Type this character while in a menu prompt to rotate around the lines of it. */);