Reported by Urs Liska: https://lists.gnu.org/archive/html/bug-lilypond/2017-02/msg00020.html
Italic fingerings are often used to e.g. distinguish between composer’s and editor’s fingerings. Currently, there is no way to achieve that within the default font, so italic versions of the Feta numbers should be created so that it just requires overriding font-series
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That would be nice. I think these italic numbers might also be used for tuplet numbers, clef modifiers, ottava numbers etc.? See also http://lilypondblog.org/2014/02/feta-and-bravura/ I’ll try to play with the metafont code but probably we would need an expert (and some suggestions for glyph forms; maybe not all numbers should just be slanted).
I believe that none of the glyphs in question could simply be slanted.