This exhibit is most interesting. The anode is left bright suggesting a low power device but the filament is an oxide coated ribbon that takes over 1.5 Amps to heat. The main interest is that the two grids pass in front of the filament as a pair in the same plane.
In the mid 1930's when automatic volume control was being introduced a special valve was designed in America for the purpose. The description of the two grids in the Wunderlich valve matches the design of this exhibit.
The box anode seen side on.
Here the filament supports can be seen fixed to the top mica. The ribbon filament is easy to see.
The grid wires can be seen to be a pair at each transit across the filament.
This image shows that the two grids are in the same plane across the filament.
The balloon envelope is 60 mm in diameter, and excluding the B5 base pins is 136 mm tall.