The 866A is a high voltage half wave mercury vapour rectifier. A pair in a full wave configuration feeding a choke input filter can produce 500 mA of DC at 3,000 Volts. Serious power and the rectifiers will produce damaging UV radiation and must be screened.
The base cap carries the Westinghouse name as well as the Type designation of 866A/866 showing that the improved version is suitable for replacing the original design.
The filament is contained within the shielding cylinder. The anode is the plate at the top.
The anode lead is enclosed in a glass tube for most of the length.
The classic envelope is 61 mm in diameter and, excluding the UX4 base pins, is 150 mm tall.
Reference: Datasheet. Type 866A was first introduced in 1938.