The STC 4242A triode is a near equivalent of the American Type 211A and is designed for audio power amplification. A single valve operated in Class A can produce 18 Watts without distortion. A pair in Class B, for public address amplifiers, would produce up to 200 Watts.
The envelope has the Type etched into the glass.
The support and heat sink flanges are in the centre of the working faces with the grid passing either side of the thoriated tungsten filament. The top support is ceramic and the odd angle of the filament tension springs is due to the fact that the filament is fractured in to several pieces.
The main electrodes seen from the pinch. The filament would pass through the grid four times.
The wide glass tube envelope is 58 mm in diameter, and excluding the base pins is 180 mm tall.
Reference: Datasheet. Type 4242A was first introduced in 1938.