The PM1HL is a 2 Volt amplifier triode. This exhibit has a plain glass envelope and our other exhibit PM1HL has external bronze metalising.
The PM1HL was designed as a detector and general purpose valve and dates from 1931. It was intended to be used in domestic broadcast receivers.
The inside of the envelope is blackened and this obscures the electrodes but looking upward from the base the anode can be seen as a rectangular box mounted horizontally. The filament is probably a single flat V.
The top of the envelope carries the identification, plus filament rating and maximum anode potential.
The balloon envelope is 41 mm in diameter, and excluding the B4 base pins, is 76 mm tall.
References: 1931 datasheet & 1043. Type PM1HL was first introduced in 1931.