\version "2.18.2"

Prefix = \markup {
  %% uncomment/comment these lines for C, C slashed, B or B slashed prefix :
  \roman C
  %\combine \roman C \translate #'(0.65 . -0.25) \override #'(thickness . 1.2) \draw-line #'(0 . 1.8)
  %\roman B
  %\combine \roman B \translate #'(0.65 . -0.25) \override #'(thickness . 1.2) \draw-line #'(0 . 1.8)
  %%%%%%%%%%%%
  \hspace #0.2
}

#(define-markup-command (prefix layout props string-qty) (integer?)
    (interpret-markup layout props 
      (case string-qty
            ((2) #{
                    \markup { 
                      \override #'(font-family . typewriter)
                      \concat {
                      \raise #.15
                      \fontsize #-6
                        {
                          \raise #.5 "1"
                          \hspace #-.2
                          \raise #.2 "/"
                          \hspace #-.2
                          "3" 
                        }
                        \Prefix
                      }
                    }
                 #})
            ((3) #{
                    \markup { 
                      \override #'(font-family . typewriter)
                      \concat {
                      \raise #.15
                      \fontsize #-6
                        {
                          \raise #.5 "1"
                          \hspace #-.2
                          \raise #.2 "/"
                          \hspace #-.2
                          "2" 
                        }
                        \Prefix
                      }
                    }
                 #})
            ((4) #{
                    \markup { 
                      \override #'(font-family . typewriter)
                      \concat {
                      \raise #.15
                      \fontsize #-6
                        {
                          \raise #.5 "2"
                          \hspace #-.2
                          \raise #.2 "/"
                          \hspace #-.2
                          "3" 
                        }
                        \Prefix
                      }
                    }
                 #})
            ((5) #{
                    \markup { 
                      \override #'(font-family . typewriter)
                      \concat {
                      \raise #.15
                      \fontsize #-6
                        {
                          \raise #.5 "5"
                          \hspace #-.2
                          \raise #.2 "/"
                          \hspace #-.2
                          "6" 
                        }
                        \Prefix
                      }
                    }
                 #})
            (else 
             #{ \markup\Prefix #}))))

%% example:
\markup \prefix #3