The U301 is an efficiency diode for television line scan. In use it returns energy from the circuit inductances back to the HT rail and hence to the reservoir capacitor.
The peak inverse voltage is 4,500 Volts, the peak anode current is 600 mA with a maximum rectified current of 120 mA. The heater needs very good insulation in these rectifiers and for the U301 the maximum heater - cathode voltage is 4500 Volts.
The U301 has a 200 mA heater and the U191 has the same ratings but has a 300 mA heater.
The anode is folded into an I shape. This view is of the closed end.
The heater is heavily insulated and passes through the cylindrical cathode four times. At the top of the image can be seen an insulated wire helix. This is additional separation between the heater and the wall of the cathode. It is not connected to either the cathode or the heater.
The other end of the the one piece folded anode. The inner surface is bright metal.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 73 mm tall.
References: Data-sheet, 3002 & 1040. Type U301 was first introduced in 1953.