The ED500 is a shunt stabiliser triode with a 6.3 Volt heater for parallel connected heaters, the PD500 has a 300 mA heater for use with series connected heaters and was used to stabilise the EHT supply in colour televisions. The ED500 was probably used for the same function in professional monitor equipment where isolation from the mains by way of a mains transformer was more important than the minimum cost requirement for consumer equipment.
The dissipation is high because of the very high anode voltage.
The heater, cathode and grid lie horizontally at the base.
The wide shaped cylinder is the screen connected to pin 2 and also pin 7.
The wide glass tube envelope is 38 mm in diameter and, excluding the B9D base pins, is 105 mm tall.
Reference: Datasheet. Type ED500 was first introduced in 1966.