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.
Additional lettering is also etched into the dome of the envelope.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 115 mm tall.
References: Datasheet, 1040 & 1043. Type U12 was first introduced in 1931.