The N359 is the M-OV version of the PL81 and the glass shows both Type designations. The GEC brand was reserved for valves for professional applications. The N359/PL81 would run very hot in service and for this valve the envelope has been widened by 4 mm to separate the anode from the glass a bit more and thus reduce envelope temperature.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
The high cathode power of the PL81 design made it suitable for use as a series pass element in voltage regulators, a circuit type with particular demands on the cathode.
The GEC brand, BVA logo and ugly word foreign for the valve's manufacture.
The valve had been used and the envelope was dirty. Some of the grime had to remain to avoid removing the paint.
The thin glass tube envelope is 24 mm in diameter and, excluding the B9A base pins, is 71 mm tall.