The PL504 is a purpose designed television pentode line output valve. It was introduced as a replacement for the PL500. This exhibit is of unknown manufacture but was sold by the reseller Solus.
The peak anode voltage is quoted as 7,000 Volts and shows why the line output stage of a television receiver was the last consumer electronics product to remain a valve. The CRT excepted of course.
TV line output valves found their way into amateur radio transmitters for service at HF (up to 30 MHz) because of their peak voltage characteristics and relatively high current capability.
The control grid has heat radiating fins on the top of the copper supports. Inside the anode the beam plate can be seen, although the design is given as a pentode. many power pentodes were in fact made as beam tetrodes.
The radiators at the top of the control grid are in fact U shaped for extra surface area.
The wide glass tube envelope is 28 mm in diameter, and excluding the B9D base pins, is 92 mm tall.
References: Data-sheet & 1040. Type PL504 was first introduced in 1961.