The U12 is a directly heated full wave rectifier and it was a popular design that was also produced by other manufacturers. The twin anode structure can be seen to be made of mesh and supported by a central column rather than mica sheets. This style of assembly predates the mica sheet form of construction.
Before cleaning this exhibit looked to have the U12 etched into the glass but under the grime the lettering was on the surface and some has been lost.
The remains of the Osram label on this M-OV valve.
The central glass support has stout rods that join with the pinch supports to hold the flange of mesh on the walls of the anodes that face the centre. The other supports hold the filament tension springs.
The filament is an oxide coated ribbon inverted V. The anode mesh is welded to the main support.
With some back lighting the shape of the filament within the anode can be seen.
The balloon envelope is 57 mm in diameter and, excluding the B4 base pins, is 120 mm tall.
References: Datasheet, 1040 & 1043. Type U12 was first introduced in 1931.