In addition to use as an audio amplifier in transformer-coupled circuits, the L210 was also suitable as an RF oscillator for superhet applications. This was before the advent of composite frequency changers.
Basically an improved DEL210, the M-OV type L210 was one of the last M-OV types to be made using the 'azide' process. Later samples of the L210 had conventional oxide-coated filaments which meant that they could be constructed in vertical format using mica spacers. This gave improvements in performance and life as well as a big reduction in microphony.
Manufactured from 1929, the electrode structure can be seen under the mirror deposit of the magnesium gettering at the top of the balloon. It lays horizontally with support struts at both sides of the open box like anode cavity.
The etched Marconi Valve can be seen.
The horizontal anode box with the blackening of the glass either side of the open ends is typical of the Azide process.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 79 mm tall.
References: 1040. Type L210 was first introduced in 1929. See also1929 adverts.