The U81 is a full wave rectifier with anode ratings of 500-0-500 Volts and a maximum rectifier current of 150 mA. The maximum reservoir capacitor is 16μF. The series resistance must be 100Ω or greater.
One of the stitched and blackened anodes seen full face.
The anodes seen end on.
The paper Osram label.
The wide glass tube envelope is 32 mm in diameter, and excluding the B8B base pins is 70 mm tall.
Reference: 1040. Type U81 was first introduced in 1950.