The PM stands for Philips-Mullard and were introduced in 1925. The PM2A is a later addition to the original range from 1929. The series all had oxide coated filaments.
These audio valves could drive a small loudspeaker. The PM2A had an output rating of 0.15 Watt at 5% distortion.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
References: 1931 data-sheet & 1043. Type PM2A was first introduced in 1930. See also 1930 adverts.