The 12AX7 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. This is an american design that was copied by other companies. Technical Trading are a reseller.
Mullard describe the ECC83 (=12AX7) as a high μ double triode, having separate cathodes, primarily intended for use as a resistance-coupled amplifier or phase inverter.
The 12AX7 Type designation and the triodes end on to the camera.
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.