>
> I couldn't find one.  I tried the following, but it didn't work:
>
> (set-char-table-extra-slot glyphless-char-display 0 "�")
>
> It didn't show the diamond (or, in my case, a character similar to a 
> question mark in inverse video), which is the expected behavior 
> according to the manual, but it didn't even show the diamond in a box 
> (or square brackets), instead just a empty box appeared, which is 
> unexpected.
>

Eli already explained (twice IIRC) that you can only use ASCII characters 
there, and that's also what the docstring of glyphless-char-display says: 
"its value should be an ASCII acronym string".