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.
The U281 is a Mazda design and Bentley is a reseller. In the early 1950s Mazda rectifiers suffered from flaking cathodes that caused catastrophic valve failure.
The anode area is increased by the use of a central rectangular box with side flanges at right angles.
The heaters are in series. The tape connects to the cathode.
The classic envelope is 38 mm in diameter, and excluding the IO base pins is 59 mm tall.
Reference: 1040. Type U281 was first introduced in 1950.