1876 (2nd Sess., 1875–6) 3 4C=16 vib means C in the Sub-Contra Octave, or double under-accented or under-lined Great Octave.
1956 H. Shanet (new ed.) App. 168 For the extremely low notes of the Contra Octave and Sub-contra Octave, a similar method is used to avoid some of the leger lines.