BSA Standard sold STC valves during 1926. The P425 is a battery output triode and some operating details are printed on the box. What is not given is the output power.
Through a gap in the gettering the electrodes can be seen. The anode is a flattened cylinder with two side plates holding it. The side plates pass down to a collar around the pinch stem. The collar has small crimps around the circumference that probably allow some 'give' to enable the collar to be a tight push fit onto the cylindrical pinch stem.
The anode retaining collar is just visible on the left. The anode is a flattened cylinder and the control grid has rounded edges that match the anode. The filament is a single inverted V that is held in tension by the spring fixed into the glass bead at the top.
Here the anode support plates can be seen to have turned-up edges that add considerably to the stiffness of the structure. The helical spring has a hook at the lower end to hold the thoriated tungsten filament.
This image shows the filament and the tension spring more clearly.
The balloon envelope is 59 mm in diameter, and excluding the B4 base pins is 100 mm tall.
Reference: 1043. Type P425 was first introduced in 1926.