The PM1HF is part of the first series of Philips-Mullard valves. The filament has an oxide coating applied by the Azide process. The PM1HF was introduced in 1926 as an HF amplifier and Also for use as a Detector. The µ or amplification factor is 15.
The glass at the top of the balloon is etched with the main information relating to the valve.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 74 mm tall.
References: 1931 data-sheet & 1043. Type PM1HF was first introduced in 1926. See also 1926 adverts.