The G3412 is a pair of indirectly heated rectifiers in a single envelope. The rating is 250 Volts at 120 mA and the heater is designed for series heater chains.
As a pair of indirectly heated rectifiers it is possible to configure this valve for use as a half wave rectifier with both sections in parallel, a full wave rectifier by strapping the cathodes together or even a voltage doubler.
In 1936 the retail price was 10/6. It is listed in the data booklet but not given a page of description. It is only described as a double rectifier.
The mesh anodes are wide but small front to back.
The wotking part of each section, the cathode, is placed at one end. Here both cathodes can be seen towards the centre of the valve.
One section: the cathode is at the bottom of the picture with the pair of supports seen centre and top. The central support is also the anode connection.On the right is the top mica.
A close-up of the cathodes. The left part of the image also shows the heater connections passing into what appears to be a solid cylinder of insulation. Good heater to cathode insulation would be required, especially if the valve was used as a voltage doubler.
The classic envelope is 42 mm in diameter, and excluding the B7 base pins is 94 mm tall.
References: 1043 & 4046. Type G3412 was first introduced in 1934.