The Pen383 is a beam tetrode audio output valve on a Mazda Octal base that was designed for receivers operating without an isolating mains transformer. The design was released in 1938 with other valves to build superhet receivers. The Pen383 could produce just under three Watts of audio.
Thanks to Derek Taylor for sending these images.
The box has a printed sheet pasted on that gives the basic operating conditions.
The logo and makers name.
The classic envelope is 54 mm in diameter, and excluding the MO base pins is 120 mm tall.
References: data-sheet & 1043 Type Pen383 was first introduced in 1938.