\version "2.18.2"

\score {
  \relative {
    
    
  s2.*8 %42

\mark \markup{\box B } s4\f s2.

s2.
  }
  \layout {
    \context {
  \Score
  \override RehearsalMark #'self-alignment-X = #-1
  }
  }
  \midi {}
}