The PM1LF was part of the first set of Philips-Mullard valves. The design of the PM1LF AF triode, dates from 1926. The box anode is mounted horizontally in the upper part of the envelope.
The Type designation is a large 1 and a smaller LF. The PM is not present. Below is the final BBC stamp. The bright anode is horizontal above the pinch.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 83 mm tall.
References: 1931 datasheet & 1043. Type PM1LF was first introduced in 1926.