font issue: border glyph for default menu

[E. Pacholleck]

working with grub-2.02~beta2 on linux mint-18.3
(additionally source downloaded and unpacked)

I am searching for that glyph(s) which grub uses for the borders in a
default menu. If I use my own grub font and only this font is loaded
(lsfonts showing only my font) then the border lines are substituted by
the builtin question mark (grub-core/font/font.c:81ff). This means my
own font does not have a glyph for the border line characters, nor are
there other fonts loaded (like unicode.pf2) to offer a substitution.

As nearly every source font has the ascii range, those glyphs must be
beyond ascii range, I guess somewhere in the range beginning with 
U+2500 (line symbols, since unicode-1.1). I am no c-programmer, so
I am lost when trying to find that in the sources. Would someone more
knowledgable be so kind to point me into the right direction?

Thanks very much for your attention. E.Pacholleck