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 lettering is feint but has three parts. The PM1 is separate from the LF on the other side. Also present is the aerial, coil and ground symbol logo. This image has the LF visible.
The horizontal anode. The production methods in the Azide process blacken the inside of the glass.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 84 mm tall.
References: 1931 datasheet & 1043. Type PM1LF was first introduced in 1926.