This AU3 is a UK Army rectifier with an E27 Edison Screw base cap. 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. This rectifier was made by M-OV and Cossor. The Cossor valve can be seen here.
There is another AU3 that is a full wave rectifier and fitted with a B4 base.
The single centre contact of the bayonet connection is the anode. The Ediswan Screw base has the filament connections.
The AU15 is visually the same but will have the exact filament operating values written on the bulb.
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 filament is held mechanically in folded supports as it has too high a melting point to weld.
The original box with horse hair packaging.
Thanks to Bryan Powell for sending the images.
The balloon envelope is 55 mm in diameter, and excluding the base pins is 125 mm tall.