The AC2/PenDD is a double signal diode and output pentode. The high mutual conductance of the pentode was designed to allow the single valve to operate as detector, AGC rectifier and the audio amplifier driving the speaker. The aim was to eliminate the first AF stage and thus save one valve in receivers designed for local station broadcast reception. The screw connection top cap places this valve to the early to mid 1930's, when this Type was first released.
The double diodes are a separate assembly within the envelope. The two strap anodes can be clearly seen around the oxide coated cathode. This small valve section is mounted in a glass bead and attached to the main pinch by stout wires.
The double diode on the left with the output pentode on the right. The faint lettering can be seen at the top of the image.
The output pentode. The anode is a blackened mesh box.
The screw top cap. The cathode is a rectangular box with a wide flat surface. The fine control grid is close wound and passes very near the cathode. The screen grid is not far from the control grid and the suppressor grid is close to the anode.
This image is of the later PenDD4020 but visually matches the AC2/PenDD output pentode top components.
The classic envelope is 54 mm in diameter, and excluding the B7 base pins is 131 mm tall.
References: Data-sheet, 1040 & 1043. Type AC2/PenDD was first introduced in 1934.