Rear Derailleur (10 speed)
Compatibility warning: none of the 7970 series components will work with other generation Di2 components. The plug is different and simply won't fit.
This short cage (SS) DURA-ACE rear derailleur will take a max low sprocket of 27T and a min top sprocket 11T. It has a total capacity of 32T with a max front difference of 16T.