The Mullard SG20 is a variable μ RF tetrode (screened grid valve) that was introduced for use on DC mains in 1932. The heater is unusual in being rated at 180 mA.
The anode top cap is of the screw type. Inside the metalised envelope the shape of the rectangular screening cage can be seen. The g2 has horizontal wires not a mesh and the anode consists of two plates supported from the top cap connection.
The B5 base with slotted and compressed pins.
The envelope is 49 mm in diameter, and excluding the B5 base pins is 129 mm tall.
References: 1043 & 1040. Type SG20 was first introduced in 1932.