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 anode seen full face. The pentode was assembled in a jig and the shaped wires from the pinch were spot welded to the electrodes. The copper control grid supports are held in collars that have been crimped to connect.
The box anode from the side.
The top of the control grid supports has a heat radiating fin. At the lower end (right side) the grid wires can be seen to be shaped to pass flat across the anode.
The top mica and within can be seen the three wire grids.
The bottom mica and within can be seen the oxide coated cathode.
The classic envelope is 52 mm in diameter and, excluding the B7 base pins, is 110 mm tall.
References: Data-sheet & 1040. Type MP-Pen was first introduced in 1931. See also1931 adverts.