This octave can also be designated with two uppercase letter (i.e. CC) or in the 1st octave under Acoustical Society of America system (i.e. C1). The MIDI Octave Designation System has two versions, one where note 1 (8.176 Hz) is octave "0" making the Contra octave the second octave (i.e. C2), and the other with note 1 (8.176 Hz) at octave "-2" making the Contra octave the fundamental octave (i.e. C0).
The indication 8vb (shown below) can be used to represent the same pitches without as many ledger lines.
Play Contra Octave Example (MIDI file format)