EXFO FTB-500 - Manual (Page 37)

Getting Started with Your Unit
FTB-500 29
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.