The AU3 is a UK Army rectifier. Made in this instance by Cossor. This half wave rectifier has a filament rated at 5.5 V at 1.5 A and an anode dissipation of 15 Watts and was intended for transmitter use.
The single centre contact of the bayonet connection is the anode. The Ediswan Screw base has the filament connections.
Thanks to Bryan Powell for sending the images.
The construction is similar to the R-Type in that a bright cylinder is threaded by a single strand filament. This will be pure tungsten and is thus a bright emitter. The maximum anode dissipation is 15 Watts.
The filament is held mechanically in folded supports as it has too high a melting point to weld.
The original box with horse hair packaging.
The balloon envelope is 55 mm in diameter, and excluding the base pins is 125 mm tall.