The PM2A is an early audio output valve. It is designed for battery operation and, naturally, the output power is very low. The anode load is quoted as optimum at 7,000 Ohms.
The balloon envelope is 44 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: 1931 data-sheet & 1043. Type PM2A was first introduced in 1930. See also 1930 adverts.