The Cossor 53KU is an indirectly heated full wave rectifier and this example looks to be from an early batch and is of greater diameter than the later 53KU.
The reservoir capacitor should not exceed 16μF and the valve needs a minimum effective series resistance of 75 Ohms to protect it from excessive peak currents.
The construction is typical with two separate valve sections in the envelope. The cathode is tied to one side of the heater and this requires a well insulated independent winding on the mains transformer. In fact it was normal to rate rectifier valves at 5.0 Volts when other valves ran at 6.3 Volts to ensure that the rectifier was powered separately. The anodes are pressed to shape and stitched together enclosing the anode support rod.
The top mica shows the oval cathodes and the insulated heater wires that pass the length of the cathode four times.
The classic envelope is 56 mm in diameter, and excluding the IO base pins is 122 mm tall.
References: Data-sheet & 1040 Type 53KU was first introduced in 1958. See also1958 adverts.