The P425 battery output triode from Lissen requires a load impedance of 5,000Ω and can deliver 800 mW of audio.
The Type designation is etched into the dome of the balloon envelope. The gettering obscures most of the inside but at one angle the glass bead support is visible as is the vertical anode with flat sides and rounded ends.
The ribbed anode can be seen as can the glass bead that holds the filament tension springs. The under side of the bead holds a support that descends to the pinch. Also present is a mica sheet that holds the other electrodes in position. Clearly this was made at the cross-over between the glass bead/rod supports and the mica sheets.
The balloon envelope is 41 mm in diameter, and excluding the B4 base pins is 96 mm tall.
Reference: 1043. Type P425 was first introduced in 1926.