The type 77 is a general purpose pentode voltage amplifier/detector. As a high impedance device the control grid is taken to the top cap for maximum isolation from the anode circuit.
The Type 77 is a sharp cut-off pentode similar to 6J7G.
The outer perforated screen cylinder, and below is the lower dish screen. Apart from the heater wires there is not much to see. It looks as if the four pillars presented on the front label are those holding the screen electrode et al to the pinch.
The top of the screen has the mica pieces that hold the internal assembly firmly to the shoulder of the envelope.
The anode cylinder and wire grids can be seen within the screen.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 99 mm tall.
References: Data-sheet, Data & 4059. Type 77 was first introduced in 1932. See also1932 adverts.
