This is a magic eye tube/valve. It has an 8-pin base which fits an 8-pin subminiature socket.
The magic eye tube (or valve) for tuning radio receivers was invented in 1932 by Allen B. DuMont.
The earlier types were end-viewed (see the EM34), usually with an octal or side-contact base. Later developments featured a smaller side-viewed noval B9A based all-glass type with either a fan type display or a band display (see the EM84). The end-viewed version had a round cone-shaped fluorescent screen together with the black cap that shielded the red light from the cathode/heater assembly. This design prompted the contemporary advertisers to coin the term magic eye, a term still used.
There was also a sub-miniature version with wire ends (Mullard DM70/DM71, Mazda 1M1/1M3, GEC/Marconi Y25) intended for battery operation, used in one Ever Ready AM/FM battery receiver with push-pull output, as well as a small number of AM/FM mains receivers, which lit the valve from the 6.3V heater supply via a 220 ohm resistor or from the audio output valve's cathode bias. Some reel-to-reel tape recorders also used the DM70/DM71 to indicate recording level, including a transistorized model with the valve lit from the bias-oscillator voltage.
(Wikipedia)
