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 used a B4 base cap with a side terminal that connected to the screen grid g2. In 1929 this B5 based version was introduced.
The top of the envelope shows filament and anode voltage, as well as identifying the valve. The double vision effect is the reflection from the inner surface of the envelope that is silvered due to the firing of the magnesium getter during final manufacture.
The balloon envelope is 47 mm in diameter and, excluding the B5 base pins, is 86 mm tall.
References: 1931 datasheet & 1043. Type PM24 was first introduced in 1928.