This ML4 looks to be an early example as it is enclosed in a balloon envelope. We also habe the later shouldered or classic style of envelope ML4.
Although sometimes listed as a triode output valve, the ML4 was the definitive descendent of the famous (or infamous) Type KL1 of 1926 and was primarily a professional and military type, little used in domestic radios.
The ML4 was quite widely used as an RF oscillator, as a single-valve phase splitter, and as a driver for larger valves.
The Marconi paper label.
The mesh anode seen from the side.
Close-up view of the electrodes. The construction pre dates the use of mica sheets as supports and a glass rod is the anchor point for the top of the supports.
The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 105 mm tall.
References: Datasheet, 1040 & 1043. Type ML4 was first introduced in 1929.