The MSG/HA is a metalised screen grid valve designed for RF amplification. It features the original standard heater for UK AC valves of four Volts at one Amp.
The 1931 advert shows the paper label with the New Process flag. This exhibit has the same label and a screw top cap. This dates it to between 1931 and around 1934 when the top hat top cap became universal.
The gettering obscures the inside.
From below the pinch can be seen. Above the pinch the oxide coated cathode is a flattened cylinder. The control grid winding is hidden inside the lower screen. The screen grids are unusual in that a separate wire grid is placed either side of the central grid and cathode. These two grids connect to the screen ring that can be seen above the pinch. The anode can not be seen.
The balloon envelope is 44 mm in diameter, and excluding the B5 base pins is 130 mm tall.
References: Datasheet, & 1043. Type MSG/HA was first introduced in 1931.