Yeah... probably RD-R8050-GS.
If so, all of these are compatible:
- RD-6870-GS (SS would work too... but that's that small cage)
- RD-9070 (small cage)
- RD-R8050-GS (and SS, but that's the small cage)
- RD-R9150 (small cage)
- RD-RX815 (GRX, but compatible)
Note that the 'M' family will not work (M8050, M9050), and 6770 and 7970 also won't work.
Also... R8150 and R9250 are 12-speed Di2, which is not compatible with 11-speed Di2
