The PM3 was introduced in 1926 as a general purpose triode with a μ of 14. The filament current, initially 0.1 Amp was later reduced to 0.075 Amps as in PM3. This exhibit draws 0.1 A at 4.0 V.
This exhibit has the third type of BBC stamp etched into the glass. This dates this exhibit to between the 1926 introduction and 1927 when the use of the stamp was discontinued.
The Mullard logo of aerial, coil and ground is also present.
The final etching is the Mullard name logo and additional details. These three images of the envelope proved tricky to photograph due to their faint nature and an oblique angle was required that gave rise to only a part of the image being in focus.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 94 mm tall.
References: 1931 datasheet & 1004 Type PM3 was first introduced in 1926.