Type 6BN6 is a late 1940s 'gated beam' FM discriminator. Where a relatively cheap quadrature coil can be used in place of more expensive wound components that are required for other discriminator circuits.
A diagram of the electron path in a 6BN6 gated beam valve.
Looking along the electron path, the heater and cathode is in the foreground.
Looking side-on to the electron path. The heater/cathode is at the bottom of the image.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 57 mm tall.
References: Data-sheet & Brian Cuff. Type 6BN6 was first introduced in 1965. See also1965 adverts.