Shimano Ultegra ST-6870 Dealer's Manual page 46

CONNECTION OF THE ELECTRIC WIRES
Connection to the Dual control lever
< SW-R671/SW-9071/ST-6871 >
SW-R671
SW-9071
ST-6871
< SW-R610 >
SW-R610
46
For the following models, attach to the
electric cable (the type that comes out from a
component) junction (A) of the product.
SW-R671/SW-9071/ST-6871
The port shape is different only for SW-R610.
Only ST-6871 is supported