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. The B7 based version followed in 1933.
AC/Pen has a bright anode and the B5 base whereas this version differs in that the anode is blackened for better heat radiation.
Side view of the anode and also the g2 side terminal on the base. The latter has lost the screw top.
The identification is etched into the dome of the envelope. Within the anode can be seen the three wire grids. The control grid is quite hard to make-out but the wire can be found passing across the oxide coated cathode.
The major supports slot into tubes that are sealed within the pinch. The control grid winding is fixed to a single support rod and on top of that rod is a heat radiating cylinder of blackened metal.
The pinch components. The fixing of the control grid can clearly be seen in this image.
A close-up of the lower mica components.
The balloon envelope is 58 mm in diameter and, excluding the B5 base pins, is 116 mm tall.
References: Datasheet, private communication & 1043. Type AC/Pen was first introduced in 1930.