The HX966B is a half wave rectifier with the anode at the top cap. It is probably a mercury vapour rectifier but no mercury spots can be seen on the envelope. The 966 is a MV rectifier and a close relative of the 2V/400A.
At www.armyradio.co.uk is mention of four HX-966 rectifiers being used in the WWII Japanese Model TM Short Wave Mobile Transmitter, Improvement 3. This was a Short Wave transmitter with an output of 400 Watts. As a portable set the low voltage drop across a MV rectifier would be important in reducing power requirements.
The etched lettering on the envelope including the Type designation. The electrode construction looks like a mercury vapour device.
The balloon envelope is 62 mm in diameter, and excluding the UX4 base pins is 154 mm tall.
References: Observation & Internet.
Updated July 14, 2014.