The 65W4 is an American half wave rectifier.
No details have as yet been found for this rectifier. Examining the Type designation gives a heater rating of 65 Volts and the final 4 indicates that the visual inspection of the valve as an indirectly heated half wave rectifier is correct. The implication is that this was destined for equipment without a mains transformer and a series heater chain. The rectifier taking half of the mains voltage.
The outer side of the anode plates are blackened, and the inner surface, seen on the other picture, is left bright because the heat generated by electron bombardment has to be taken away not efficiently radiated back to the cathode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 58 mm tall.
Updated March 20, 2014.