The PM24B is a directly heated output pentode capable of some 3.0 Watts output. The PM24B was introduced in 1930.
The type designation and other information is etched into the top of the envelope.
Seen through the dome of the envelope the horizontal electrode assembly can be seen.
The balloon envelope is 57 mm in diameter and, excluding the B5 base pins, is 105 mm tall.
References: 1931 data-sheet & 1043. Type PM24B was first introduced in 1930.