The Pen141 is a Mazda battery audio output pentode with an output of 240 mW.
This Pen141 has no markings left on the envelope, they have been rubbed off. The clue to the identity of this pentode started with the Mazda Octal base and the missing pins. Visually it is clearly a low power pentode with wire grids. Checking the pin connections of MO valves narrowed the search. Visual inspection of the wires passing through the pinch completed the matching process.
The broad side of the anode. All of the connections and electrode connections and supports are visible.
Looking through the bottom mica into the anode. The suppressor grid is connected to the negative end of the filament and can be seen as a wide pitched wire closer to the anode than the other grids. The screen grid has its wires passing flat across the inner grid.
Looking into the side of the anode. The suppressor grid is seen as a flattened helix. The inner grids are hard to see without the full sized hi-resolution image but the photograph is focused on the oxide coated filament wires than can be seen as vertical lines.
The flattened aspect of the suppressor grid together with the flat shape of the screen grid. The control grid is obscured by the anode support rod in the foreground.
A general view of the grids. This layout is typical of many general purpose pentodes.
This image was donated by Han and shows that the valve was made in America under the Lend Lease scheme of WWII and distributed in the UK under the Mazda brand. The Pen141 identification can clearly be seen.
The classic envelope is 32 mm in diameter, and excluding the MO base pins is 70 mm tall.
References: Datasheet & 1043. Type Pen141 was first introduced in 1939.