The U281 is rated as an HT rectifier and as an efficiency diode for TV use. It is designed for a maximum inverse voltage of 3,000 Volts and a peak current of 600 mA. For HT use the maximum reservoir capacitor is given as 16 μF.
This half wave rectifier looks like a full wave rectifier but both sections are connected internally in parallel.
One anode face view. The anode is formed in one piece with a round area on the right and the flat side is stitched together.
The heater and cathode of the front section is on the left near the stitches.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 75 mm tall.
Reference: 1040. Type U281 was first introduced in 1950.