The 12BY7A is a 1950s video amplifier output valve designed for boosting wideband TV video signals to a sufficiently high level to be transmitted over a considerable length of cable without loss of quality. It was also used in professional test equipment such as Tektronix oscilloscopes.
The heater is rated for 12.6 volt use and is centre tapped.
The bright shape is the beam forming plate.
The grids pass flat across the face of the cathode and have been precisely formed. The top mica is a double mica and by doubling the thickness of the mica supports the valve could be made more resistant to vibration.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 60 mm tall.
References: Datasheet, 1040 & Henning Hertz. Type 12BY7A was first introduced in 1955.