The KT36 was designed as a line output valve for television use and this Marconi branded valve looks to be unused.
The maximum positive surge on the anode is 4,000 Volts, and the dissipation is 10 W for the anode and 3 W for the screen grid.
The reverse carries the M-OV generic identification. One production line and three brands to be supplied (Marconi, Osram and GEC).
The cathode is both rectangular and heated by four insulated wires. The anode is rectangular and additional cooling fins can be seen on the working face. Additionally the control grid is wound on copper supports.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 113 mm tall.
References: Datasheet, 1040 & 3002. Type KT36 was first introduced in 1948.