The MX40 is a 1934 Osram heptode frequency changer. The British radio industry was slow in adopting the superhet for a variety of reasons one of which was that British valve manufacturers were slow in developing the specialised types of valve required in an efficient, low-cost set. The American radio industry showed the way and the MX40 is simply a home-brewed copy of the American 'pentagrid converter.
The oscillator voltage was designed to be 10.0 V peak to peak.
The five wire grids. Grid one is the local oscillator control grid with grid two, just two vertical rods, as the oscillator anode. Grids three and five are screen grids and the signal is coupled from the top cap to grid four.
The Marconi brand paper label.
The wire grids seen from the base. The oxide coated cathode is the white tube.
The Type designation is contained in the feint etched lozenge on the left of the picture.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 112 mm tall.
References: Datasheet, 1040 & 1043. Type MX40 was first introduced in 1934.