The 6X4 is a miniature HT rectifier following the normal pattern for a full wave device in that a common cathode is employed.
At 325-0-325 input to the rectifier this will give an HT rail higher than is required for most domestic receivers. Use for audio amplifiers and small transmitters is most probable, or driven at 250 volts for radio receivers. The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 59 mm tall.
These valves saw duty in many of the smaller classic hifi amplifiers and guitar amplifiers, however, here's a list of some exciting exotica that also uses them:-