Shimano RD-6870 Di2 Ultegra Rear Derailleur (Med. Cage) - Working, Read Desc.

Description

This rear derailleur has been tested and is fully functional.
However, it is a little quirky in that it sits a little farther inboard than normal, so you may run out of micro-adjustments before it is correctly indexed (mine used 14 of my 16 outboard micro-adjusts before getting it shifting perfect).
Hence, If you do run into the off case where you can't get it indexed with the available micro-adjustments, I have supplied some washers that, if used on between the hanger and the derailleur mounting bolt, will move the derailleur body outwards by either 1mm or 1.4mm which will be more than enough to get it indexed properly (each micro-adjust step moves it 0.2mm, so that's 5-7 extra steps you get).
(I recommend not using both washers at the same time as that reduces the number of threads engaged)