The 6U4GT is a television efficiency diode from the early 1950s for use within the line timebase circuit. The diode allows energy from the collapsing magnetic field in the line coils to be fed back to the HT rail. The maximum peak inverse voltage for this stressed component is 3,850 Volts.
The etched Type designation in an octogon as found on RCA made valves.
Typical half wave rectifier construction with central cathode tube and an anode that is dark to radiate heat and much wider than deep.
The wide glass envelope is 29 mm in diameter and, excluding the IO base pins, is 77 mm tall.
References: Data-sheet & 1040. Type 6U4GT was first introduced in 1952.