This exhibit is quite unusual in that there is a pinch at each end of the envelope. The valve is a half wave rectifier and the separation between the filament and anode connections suggests a high voltage and low current use. The screw top cap points to a 1930's design. If the E number is the type designation, then this was an experimental valve probably designed for a military application.
In Wireless World June 4, 1937 there is a short article announcing a new Osram Photocell. This photocell has the same envelope configuration as this rectifier. We can, therefore, place this exhibit at about 1937.
The valve turned through 90 degrees shows the top filament support is kept away from the anode cylinders.
The anode is formed from a sheet bent into two open cylinders with a single filament threaded through each one.
The filament is thoriated tungsten.
The spherical centre section of the envelope has etched into it the fact that it is an Osram Valve and it was made & patented in England.
The balloon envelope is 56 mm in diameter, and excluding the B4 base pins is 148 mm tall.
Reference: Observation. Type E388 was first introduced in 1937.