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 anodes are folded and stitch on one side. This flange provides additional heat sinking and a point to attach the support.
The central column is a support for the getter holder.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.
References: Data-sheet, 3002 & 1040. Type 12AX7 was first introduced in 1947. See also 1947 adverts.