The U78 is a miniature HT full wave rectifier with two anodes around a common cathode assembly.
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.
See also12X4 for the version with a 12 Volt heater.
The two anodes are identical and mounted in an X formation. The upper anode is mounted with the side legs pointing down and the lower anode has the legs vertically. In this way the maximum radiating area is fitted into the small envelope.
This image shows the face of one anode and the side of the other.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 58 mm tall.
References: Datasheet & 1040. Type U78 was first introduced in 1952.