EXFO FTB-7000 OTDR Series for FTB-500 - Manual (Page 31)

Getting Started with Your OTDR
OTDR 17
Inserting and Removing Test Modules
8. If you are using a larger or heavier module, use a front module lock to
hold them securely in place. Simply place the retaining part against the
module, then screw in the holding pin.
When you turn on the unit, the startup sequence will automatically detect
the module.