EXFO FTB-2 - Manual (Page 274)

Maintenance
264 FTB-2 and FTB-2 Pro
Replacing Batteries
6. Place the battery compartment on the unit, making sure that the sides
of the compartment are flush with the back of the unit. If necessary,
slightly move the battery compartment door until alignment is correct.
7. Lift the mobile part of the compartment screws and, while applying
slight pressure, turn the screws clockwise until they are tightened.