The PX4 is one of the classic M-OV audio power triode valves and this one is branded Osram. This exhibit has the treated anode that is seen in later versions. The filament is oxide coated by the later processes and not the messy azide process. The presence of mica discs rather than glass rods to hold the electrodes is indicative.
The filament is a single wire formed vertically within the anode cavity into four inverted V's.
The anode, Osram label and the printed Type designation.
The filament with the tension springs mounted on an auxiliary mica sheet. The springs are coils with the hooked end passing down through the centre of the coil.
The classic envelope is 50 mm in diameter, and excluding the B4 base pins is 123 mm tall.
References: Data-sheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.