External Battery Mount
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.
Use the long version if you need to mount this on the down tube have the space for it. The long mount lets you use an external battery and a bottle cage. The short mount is meant for small frames or frames that cannot accommodate the long mount.
All three versions are fastened using M4 x 10mm (or 16mm) screws.