The PL300 is a television line output valve designed for use within a 300 mA series heater chain. The EL300 is electrically identical with the exception of the 6.3 Volt 1.65 Amp heater designed for parallel heater operation.
The valve has a substantial box anode.
The screen grid has cooling fins above the top mica. The control grid is wound on copper support rods. The cathode tube is a flat rectangle. The low rectangular structure surrounding the inner grids is the beam forming plate.
The valve is essentially an all glass type formed on a glass disc with wires connected to the IO base pins rather than directly to pins as a B9D type would be.
The wide glass tube envelope is 38 mm in diameter, and excluding the IO base pins is 101 mm tall.
References: datasheet. Type PL300 was first introduced in 1968.