Post Office Box Plug Type : Having three or four pairs of ratio coils of (10, 100, 1000 ohms) and 16 series coils of selected double covered wire enclosed in a teakwood case with Sunmica Top. The blocks are made of brass size 1/2" (Square Rod) nicely golden lacquered and resistance values are engraved on them. The top is manufactured from Bakelite. Two tapping keys are provided for Battery and Galvanometer. All plugs are interchangeable. All coils are calibrated on Imported Testing Bridge Accuracy ±