The D63 is a double signal diode with separate cathodes. The old service code was ARDD3 and the later CV code was CV1301.
The Marconi paper label. It states that the valve was made in England under British Patents.
Note that on the side in an octagonal lozenge is the etched Type designation of 6H6GT. Is it possible that this is an RCA valve brought to the UK and re-branded? Our other Marconi D63 is of the classic shape and covered in metal paint.
The two small diode sections sitting above the pinch.
The wide glass tube envelope is mm in diameter, and excluding the IO base pins is mm tall.
References: Datasheet, 1040 & 1043. Type D63 was first introduced in 1937.