The Mazda AC/Pen, introduced in 1930 on the B5 base, was the first technically successful indirectly-heated power pentode capable of enough output to drive a moving-coil loudspeaker at good volume. In 1933 the B7 based version AC/Pen appeared.
This requirement for a powerful indirectly-heated pentode was obvious in the late 1920s, but early attempts to make such valves resulted in overheating of the inner electrodes due to the combination of heater, screen grid and anode power within a very confined space.
The Mazda AC/Pen was the landmark valve whose design overcame these difficulties at low cost and without causing other problems. Originally produced with a balloon envelope the design progressed to the classic shouldered envelope and the B7 base during the lifetime of the design.
The Mazda identification etched into the envelope dome. There are two sets of mica supports, a rectangular one holding the electrodes in place (far left) and then a second set that pass above the top mica to wedge into the top of the glass.
The balloon envelope version has a metal cylinder for the anode, whereas her the anode is made of mesh and more oval in cross section.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 119 mm tall.
References: Datasheet, private communication & 1043. Type AC/Pen was first introduced in 1930.