The PM252 was introduced in 1927 as an audio output triode for battery sets. The gm was 0.9. The first improvement was in 1929 and a second improved version appeared in 1931. The final gm was 3.7. By the 1933/4 data book the valve was not listed.
The dome gives the Type and the maximum anode volts. At 150 this will be one of the improved valves from 1929 or 1931.
The pinch and horizontal electrode assembly.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 88 mm tall.
References: Data-sheet, 4023 & 1043. Type PM252 was first introduced in 1927. See also1927 adverts.