The Pen45 is an audio output valve. This valve, designed in 1938, was Mazda's post-war answer to the ubiquitous American type 6V6G used in consumer receivers. See Pen45 for fuller details.
The Pen45 was intended as a generously rated general-purpose output valve for domestic radios, radiograms, etc. It was a very good valve and a stalwart workhorse which seldom went wrong and therefore survives in fair numbers.
The top of the bulb has the identification printing, in standard Mazda style.
The classic envelope is partly metallised on the outside and the maximum diameter is 42 mm. Excluding the MO base pins the valve is 91 mm high.
References: Datasheet, 1040 & 1043. Type Pen45 was first introduced in 1938.