The U52 is directly heated full wave rectifier with a rating of 500 Volts on the anodes and a PIV of 1,400 V. The U52 has a maximum reservoir capacitor rating of 16 µF and a minimum series resistance of 180 Ohms.
The Osram retail brand is denoted by the stick on paper label.
The twin anode sections contain oxide coated ribbon filaments that are supported at the top on tension springs.
The lozenge with the type designation has almost gone, with care the U52 can be read.
The classic envelope is 50 mm in diameter, and excluding the IO base pins, is 120 mm tall.
References: Data-sheet, 1040 & 1043. Type U52 was first introduced in 1938.