The U12 is a directly heated full wave rectifier and it was a popular design that was also produced by other manufacturers. The Marconi logo label is stuck to the side of the envelope. The twin anode structure can be made out within the silvering of the glass caused by the firing of the getter.
The Marconi label on this M-OV valve.
This early 1930s construction features mesh anodes and a central glass bead to hold the electrodes. The later U12 with the shouldered envelope uses mica discs for holding the electrodes.
The dome carries etched identification. The filaments are oxide coated ribbons shapes as an inverted V and held tight by a tension spring.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 116 mm tall.
References: Datasheet, 1040 & 1043. Type U12 was first introduced in 1931.