The MP-Pen was made in both B5 and B7 base versions, and was manufactured from 1931 in B5 and 1934 in B7. This popular type of audio output valve had many equivalents.
The given parameters include a recommended cathode resistor or 450Ω and a load resistance of 1,000Ω.
When seen from the top and through the top mica the spacing of the wire grids can be seen.
In this oblique view the notched grid support rods can be seen. The central coated cathode tube is in the centre with the control grid seen as a helix close to it. The screen grid is flattened so as to be parallel to the anode across the broad face. The final lazy helix is the suppressor grid.
The balloon envelope is 54 mm in diameter and, excluding the B5 base pins, is 110 mm tall.
References: Datasheet & 1040. Type MP-Pen was first introduced in 1931.