SHIMANO B-Joint (Di2 spec) SM-JC41 Built-in type.

For internal DI2 cable routing, SHIMANO's SM-JC41 Junction B is the central connector for DI2 systems. The derailleur, shifter, dashboard, and battery cables all feed into the SM-JC41 4-port junction, which is built into the bike frame. 

Compatibility
Dura Ace Di2 9070
Dura Ace Di2 R9150
Ultegra Di2 6770
Ultegra Di2 6870
Alfine Di2
XTR Di2 M9050 engine
XT Di2 M8050 motor