The Mullard PM254 is a directly heated low impedance triode. It was introduced in 1927 and an improved version was released in 1929.Mullard give details for the use of this triode for audio output service.
The lettering on the dome.
An attractive box to attract the consumer, made oversize to accommodate an inner protective cardboard sleeve.
The balloon envelope is 47 mm in diameter, and excluding the B4 base pins is 85 mm tall.
References: 1931 data-sheet & 1043. Type PM254 was first introduced in 1927.