The 12AT6 is a double diode triode for receiver applications. It is of medium impedance and very low current. Type 6AT6 differs only in having a 6.3 Volt heater.
The cathode tube extends above the triode structure and two bright metal straps can be seen in the picture, these are formed into 2 mm squares at the top of each strap. These are placed either side of the cathode and form the diode anodes. This construction is distinctive in identifing the 6AT6 and 12AT6.
The diode anodes seen end on reveal how thin the sheet metal is. The central triode is carbonised to radiate heat.
The helical grid can be seen at the bottom of the anode.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Datasheet & 1040. Type 12AT6 was first introduced in 1946.