The N31 is an audio output pentode for use with 300 mA heater chains and intended for single ended operation.
Internally the anode is a substantial box and contains a flat cathode tube and three grid supports. The first two wire grids are visible but not the third. The expectation is that it is a pentode as one would expect to see some sign of beam plates if fitted.
The size of the valve and the top cap connection together with the domestic Marconi label on this M-OV valve suggest that it was designed for line output stage service in early television receivers. See20P1 as a line output valve.
The side view shows the Type designation but no additional internal detail.
The final picture shows the designation stamp in detail.
The classic envelope is 54 mm in diameter and, excluding the B7 base pins, is 128 mm tall.
References: Data-sheet & 1043. Type N31 was first introduced in 1935.