The 6AQ8 was the standard double triode for use in the front end stages of FM radio receivers. It was designed specifically for the purpose.
The envelope shows that this Westinghouse valve was manufactured in Germany.
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 envelope declares this to be a premium product. No gold plated pins though.
One of the triodes. The coiled heater wire and the insulation applied to the wire are on the left and enter the bright cathode tube, The emissive coating being confined to the length within the anode.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 47 mm tall.
References: Datasheet & 1040. Type 6AQ8 was first introduced in 1954.