The 816 is a half wave mercury vapour rectifier with the old American standard 2.5 Volt heater. The filament warms up to operating temperature in 10 seconds and a thermal delay switch would be used in series with the anode circuit so that 60 Hz HT would not be applied until the valve was hot.
The voltage drop across the valve is 15 Volts.
The pinch and cylindrical anode.
The filament is supported on stout rods. The mercury can be seen on the inside of the glass. The filament is a small diameter helix.
The classic envelope is 39 mm in diameter and, excluding the UX4 base pins, is 103 mm tall.
Reference: Data-sheet. Type 816 was first introduced in 1957.