The P27/500 is the Tungsram version of the classic PX25 from M-OV.
The P27/500 is directly heated and feeding the filament to minimise hum is not straight forward. Williamson, who worked for M-OV and then Ferranti, indicated that the modern indirectly heated KT66 when strapped as a triode was almost identical in performance to the PX25.
The large anode is supported by four substantial struts all fixed to a collar that is clamped around the pinch stem. The valve has four tension springs at the top, the one on the left is raised as the filament strand is broken.
The pinch area with thoriated tungsten filament wires above. Note that the filament wire is held in folds of the supports as welding was not possible.
The balloon envelope is 65 mm in diameter, and excluding the B4 base pins is 137 mm tall.
References: Datasheet, 1005 & 1043. Type P27/500 was first introduced in 1937.