The LS5A from M-OV was a dull emitter valve designed for audio amplification. The LS series were much used by the Post Office for telephone repeater amplifiers. See POVT25.
The LS5A was introduced in 1924 and has a lower impedance than the original LS5, and so could produce more output power. Later versions of the LS5A moved to a Bakelite base with the seal concealed within.
The anode is oval and the grid is flattened within it. The filament is an inverted V and was carbonised to reduce the effects of gas contamination. Some dull emitters had a thoriated tungsten filament, but the carburised filament achieved a long life, the downside was its mechanical fragility.
In the mid 1920s the term 'dull emitter' was often used to cover a variety of production processes, all of which produced low temperature, hence dull, emission surfaces.
The balloon envelope is 53 mm in diameter and, excluding the B4 base pins, is 111 mm tall.
References: Data-sheet & 1005. Type LS5A was first introduced in 1924.