This is a storage for d-type ceramic fuses. Even in today's world of automatic circuit breakers the older d-typ style ceramic / porcleain fuses are still in use at many locations. I plan to mount one of these in near proximity of each of my three electrical boxes. Someone suggested that the slot is a nice feature that allows you to see the colour of the fuse (i.e red = 10A, grey = 16A, etc.) if the fuse is dropped in back side out but i prefer to always have them the other way as it makes it somewhat easier to grab a fuse.