The PX4 is one of the classic audio power triode valves and this one is branded Osram. The design belongs to M-OV, Marconi or Osram paper labels are used to brand the valves.
This exhibit is an early version with a blackened interior and a thin sloping anode. The electrodes are held in place by a collar around the pinch stem. The production is probably by the azide process and this would account for the blackening within the envelope.
The base cap has M-OV around the centre hole. The slanted anode is about 6 mm front to back. Later PX4 versions had a larger front to back dimension.
The etched details on the dome are very difficult to see. This image took some trial and error to make.
The sloping anode.
The classic envelope is 60 mm in diameter, and excluding the B4 base pins is 132 mm tall.
References: Data-sheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.