The Mazda 6C12 triode heptode was designed for the role of frequency changer (mixer) in superhet receivers. This double valve has a common cathode. The triode is to be used as the local oscillator feeding the 13 Volts pk-pk amplitude sine wave to the heptode mixer.
The reverse side shows the made in England lettering.
A close-up on the valve reveals that the triode is mounted above the heptode section. The triode grid can be seen through a hole in the outer screen.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 57 mm tall.
References: Datasheet, 3002 & 1040. Type 6C12 was first introduced in 1955.