The Shimano Junction Box snaps directly onto the Di2 Genie.
The Di2 Genie-6-50-10 extends 50mm under your stem, at 6 deg, and has a 10mm drop. It adds a total of 5mm to your headset stack height. It is for stems with a 6-degree rise (or more).
The Di2 Genie fits any 1 1/8″ headset.
Also available as BDop Di2 Genie 6-40-5