The 12AX7 is a high impedance amplifier triode and equivalent to the Mullard ECC83 where full details can be found. Colomor imported and distributed foreign valves.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.
References: Datasheet, 3002 & 1040. Type 12AX7 was first introduced in 1947.