The PM6 is a Philips/Mullard Azide audio output triode. This exhibit carries the third version of the BBC stamp and thus dates to about 1927. The design was released in 1926 (first characteristic table) and an improved version released in 1929 (second set of valve characteristics).
The improved valve was designed to work into an 8,000 Ohm load and produce 160 mW.
The first batches of the PM6 had a raised line down the side of the base to denote the anode pin. This line was filled with white paint. This exhibit does not have this feature.
The paper label has been applied upside down.
The box anode lies horizontally within the envelope.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 93 mm tall.
References: 1931 datasheet & 1043. Type PM6 was first introduced in 1926.