Unknown Spherical Diode Tube

I have no definitive information on this unusual tube.  It is about 2 1/2 inches long, and the bulb is about 1 1/2 inches in diameter.  The filament is a simple hairpin, presumably tungsten, with connections to the candelabra screw base.  The plate is connected to the brass cap (5/8 inches in diameter) at the opposite end.  From the construction, I would assume it was made prior to 1920.  I have a vague recollection of having seen a picture of a similar tube of British origin some years back.  If anyone knows anything about this tube, I would appreciate an email.