As the string vibrates as a whole unit and in halves, it also vibrates in thirds, producing the twelfth, the G one octave and a perfect fifth above tenor C. This is the third partial or harmonic, and the second overtone.
If you were to play tenor C at the organ and pull a 2 2/3-foot stop only, you would hear the twelfth, the G one octave and a perfect fifth above tenor C.
2 2/3' stops are fifth-sounding stops, which sound the note name a fifth above the key being played.