The 10D1 is a double diode for use on 200 mA series heater chains as would be found in radio receivers and early television receivers.
This example has a black coating inside the envelope that obscures the electrodes. Our other 10D1 has a clearer envelope.
The classic envelope is 38 mm in diameter and, excluding the B5 base pins, is 92 mm tall.
References: Data-sheet & 1043. Type 10D1 was first introduced in 1936.