The G3412 is an indirectly heated full wave rectifier from Triotron. The envelope contains two separate rectifiers and so half wave parallel operation was possible as well as full wave. Without knowledge of the heater-cathode insulation it is not possible to say if use as a voltage doubler was intended.
The actual working space is small as can be seen from this side view.
The cathodes are near the centre of the envelope with the mesh anodes extended for both support and heat sinking.
The classic envelope is 42 mm in diameter, and excluding the base pins is 94 mm tall.
Reference: 1043. Type G3412 was first introduced in 1934.