The AC/2Pen, also marked AC2/Pen, design is a 1933 audio pentode power output valve. In the 1930's Mazda held the technological lead in indirectly heated audio power valves. The 1930 landmark AC/Pen was a first generation device, and a very successful one, but required considerable drive.
The AC/2Pen was the market leading second generation output pentode. The second generation valves had better gain and could be driven directly from the detector output if required.
This valve is 10 mm greater in diameter than the AC2/Pen.
The dome carries the identification and is one of the few places to see the construction. The control grid has a circular heat sink connected to the support rods.
Looking up from the base one can see the insulated heater entering the flat sided cathode tube. The glass distorts the image as the cathode is a perfect rectangle with one heater at either end of the bottom opening. The three wire grids are also visible, all passing flat across the face of the cathode.
The classic envelope is 54 mm in diameter and, excluding the B7 base pins, is 116 mm tall.
References: Datasheet & 1043. Type AC/2Pen was first introduced in 1933.