When capolinea insulators didn't exist, these boxes were used at the end of telephone lines for single users. Inside there are the contacts, fuses, and connections of the line open wires, and the covered cable (visible below the base of the box). The box has a porcelain base and is mounted on the same crossarm of the insulators. Here in comparison with a 2/1920 pyrex insulator and opened. I know there's also another smaller size of these boxes (without fuses).