The EFM1 is an oddity. It combines a tuning indicator with an audio variable μ pentode. The attached pdf gives a very interesting description of the valve and its use. The indicator is based on the EM1 tuning indicator. This valve could save on valves in a receiver but the design was a compromise and did not replace the more conventional indicators.
The base is limited to eight pins and so two independent valves were not possible in the one envelope.
The indicator window is tinted, but internally follows normal practice of a grid around the cathode and deflector electrodes to form the shadow.
The classic envelope is 36 mm in diameter, and excluding the Ct8 base pins is 78 mm tall.
References: Data-sheet & 3002. Type EFM1 was first introduced in 1938. See also1938 adverts.