The PM4DX is a triode and this exhibit carries a paper band on the Bakelite base and has details etched into the envelope. These start with the Mullard name, then the PM4DX as well as aerial, coil and earth logo.
The PM4DX was designed for operation as a detector.
The type designation was also etched in large letters on another part of the envelope as well as the BVA logo.
Valves in the 1920s tended to be packed in magnificent boxes with inner compartments to further protect the delicate contents. As home construction was very popular the basic operating characteristics appear on the box. This was clearly a valve box made to attract the consumer.
The balloon envelope is 42 mm in diameter, and excluding the B4 base pins, is 90 mm tall.
References: 1931 datasheet & 1043. Type PM4DX was first introduced in 1928.