The 6Z5 and 12Z5 are the same valve. The heater has two sections rated at 6.3 Volts and 0.4 Amps. In series the rating is 12.6 Volts at 0.4 A and in parallel they draw 0.8 Amps. The 60 mA output rating suggests domestic receiver service.
The side view shows the flat construction of the two rectifiers above the pinch stem. The width is the diameter of the cathode and surrounding anode that form the active cylinder. The rest of the anode forms a flat wing to radiate the heat generated by the heater and electron bombardment of the anode during operation.
The anodes are carbonised for more efficient radiation and are stitched together. Flanged beads hold the anode to the mica both top and bottom.
A close-up of a single section showing the fixings to the mica.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 89 mm tall.
References: Datasheet & 1043. Type 6Z5 was first introduced in 1937.