The PX25 is one of the all time classic audio output triodes. This example is branded as an Osram product as can be seen below, note also the hand construction with the band held against the pinch.
The PX25 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 Osram brand is etched into the glass. The four support struts fitted to a collar around the pinch stem was introduced in the LS6A that became the PX25.
The Type designation is etched into the dome of the envelope.
Unlike the earlier LS6S that had coated filament ribbon the PX25 has four hairpin filaments of coated wire.
The balloon envelope has a maximum diameter of 65 mm and excluding the B4 base pins it is 144 mm tall.
References: Data-sheet, 1005 & 1043. Type PX25 was first introduced in 1932.