This is the gear position sensor. It uses a 'T' shaped interlock with the selector mechanism on the back of the gearbox (not 'D' shaped as the earlier type). This doesn't fit any twin cam models, i.e. those built after 08/2009 (see ELE18106).

If we have none of these available, the later version with the cable will fit - see ELE11960.
For a new O ring see ELA80582