The Z63 amplifier pentode was designed as a low noise audio pre-amplifier and was first seen at the Radio Show in 1937. The KTZ63 is identical.
The Marconi brand label. The Marconiphone company produced consumer goods.
The pinch at the bottom and the lower screen above. The coiled heater would cancel some of the mains hum - essential when amplifying small audio voltages.
The top screen and mica. The cathode is a round tube.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 94 mm tall.
References: Data-sheet & 1040. Type Z63 was first introduced in 1937. See also1937 adverts.