The 6L12 is a double triode designed especially for the front end stages of FM radio receivers.
The two independent valves are screened by a rectangular plate across the diameter of the envelope. One triode would be used as an RF amplifier and the other as the local oscillator. The oscillator would be run 10.7 MHz lower than the incoming signal, and in the mixer a standard 10.7 MHz IF would be produced.
The sections seen end on with the screening plate in the centre. The glass carries the Made in England and BVA identification.
One anode face view. The screen behind is larger than the triode section. The grid can be seen to be wound on copper support rods.
One triode in close-up.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 45 mm tall.
References: Data-sheet & 1040. Type 6L12 was first introduced in 1954.