The PM1A from Mullard was part of the first series of Philips-Mullard (PM) valves to be issued and comes from the two volt range. The PM1A was issued in 1927 and was designed for Resistance Capacitance Coupled (RCC) applications. The first inter-stage coupling was by transformer but as gain increased the cheaper but less efficient RCC coupling became the norm.
The lettering is etched into the dome of the balloon envelope.
Looking into the silvered envelope from the base reveals the horizontal box anode. The thin sheet metal has a pattern pressed into it to add strength and keep it in shape. From the pinch supports rise to the electrodes. The strut on the left side goes to the anode. At the top of the image the grid can just be seen.
Another view showing the rectangular shape of the box anode.
The grid can be seen on the left. It is wound between two supports and the oxide coated filament is a single V and the end connections can be seen.
The balloon envelope is 41 mm in diameter, and excluding the B4 base pins is 77 mm tall.
References: 1931 datasheet & 1043. Type PM1A was first introduced in 1927.