The PM24 dates from 1928 and later versions such as the PM24A operated at twice the anode voltage and gave 2.5 Watts of output. The PM24 is a pentode designed to operate with AC on the filament and uses the (then) standard four Volt rating.
Initial 1928 production of the PM24, as seen in this exhibit, used a B4 base cap with a side terminal that connected to the screen grid g2.
The etched identification on the dome of the envelope.
It is difficult to see the horizontal electrode assembly beneath the black remains of the Azide process. This image is distorted as the shot is almost parallel to the base and thus passes through the curved bottom of the glass.
The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 87 mm tall.
References: 1931 datasheet & 1043. Type PM24 was first introduced in 1928.