The PM24A Type is an early mains audio output valve initially released in 1929 in a balloon envelope (PM24A). With a load resistance of 8k Ohms it could deliver 1.5 Watts.
In the 1930's it was improved and the new ratings are in the second set of parameters. With a load resistance of 10k Ohms it could deliver 2.5 Watts at 10% distortion. This exhibit in a shouldered or classic envelope is of the later manufacture.
The top mica can be seen to fit tightly into the glass envelope. This produces a much more rigid assembly than was possible in the balloon style of envelope. The effect would be to reduce microphonic effects caused by vibration.
The top mica components. The three grid supports can be seen as well as the filament tension springs. This is an AC valve with an oxide coated filament and not an indirectly heated type.
Side view of the anode with the tension springs to the right.
The classic envelope is 40 mm in diameter and excluding the B5 base pins it is 94 mm tall.
References: 1931 data-sheet & 1043. Type PM24A was first introduced in 1929.