The PP3/250 was Mazda's answer to the PX4, one of the classic audio power triode valves. This probably an early version because it has the balloon shape and the electrode supports including the filament tension springs are held in a glass rod to the side of the anode. An improved version was released in 1939 the ratings given are for the improved version. The original had a maximum anode current of 40.0 mA and output power (single ended) of 2.5 Watts.
This exhibit is marked DUMMY as a production sample. However, the filament is intact and the valve probably is a fully working valve.
The oxide coated filament is a single wire formed vertically within the anode cavity into four inverted V's.
The glass support rod connects to the pinch with a single strut. The top mica supports the grid in place inside the anode.
The dome etching, top mica, compound filament supports and the oxide coating of the filament is just visible.
The pinch is surrounded by a hand tightened band that holds the main anode supports. The grid wire can be seen within the base of the anode.
The balloon envelope is 57 mm in diameter and, excluding the B4 base pins, is 119 mm tall.
References: Datasheet, 1040 & 1043. Type PP3/250 was first introduced in 1932.