The L610 is an example of an early audio output valve designed to drive a loudspeaker in a domestic radio receiver, but 70 mW is not a lot of audio power.. It dates from 1929 and was for battery operation.
Battery-operated low-impedance types such as the L610 were also used in stable, low-drift RF oscillators for driving broadcasting transmitters and similar high-quality equipment.
The etched lettering is on the dome of the envelope.
The interesting thing about this exhibit is that one can see the inner construction and the horizontal electrode assembly.
