This exhibit is an American made 813 with a Mullard logo on the front.
The 813 is a high voltage, low current beam tetrode with a thoriated tungsten filament. Like many vintage valves the 813 has been used to build audio amplifiers. A single ended and triode connected 813 would run successfully from an HT of 800-1,000 Volts.
The valve base would be mounted on spacers so that a blower could be fitted under the chassis and a stream of forced air cooling would pass over the envelope.
The etched eight sided box around the Type designation is typical RCA. The base skirt has Made in USA in clear black lettering. Looking closer to the right of those words and the name RCA in a circle can be seen.
The graphite anode with the bright beam plates within. The top ceramic support carries the filament tension springs.
The classic envelope is 64 mm in diameter, and excluding the UX7 base pins is 170 mm tall.
Reference: Datasheet Type 813 was first introduced in 1954.