The BM-DN100 external battery mount lets you use the older generation SM-BTR1 on your bike and take advantage of Synchronized Shifting - this battery mount contains updated hardware that enables Synchro-shift. If you cannot fit the BT-DN110 internal battery in your frame, this external mount and SM-BTR1 is what you'd use.
It is available in three versions, so you will probably be able to find one that suits your bike. The BM-DN100-S is the short version, supporting internal/external wiring. The BM-DN100-L is the long mount while BM-DN100-I is a long mount meant for internal cable routing. Have a look at the BM-DN100 different sizes pdf for more information.
All three versions are fastened using M4 x 10mm (or 16mm) screws.
BM-DN100-I (long, internal)