The P415 is a directly heated battery audio output triode from 1928. It is part of the unified range from BTH at a time when battery valves came in three filament values: 2 V, 4 V and 6 V. See alsoLF407 for the matching driver valve.
The P415 has an oxide coated nickel filament.
The anode is a flat rectangle and the gettering obscures most of the details.
This exhibit is identified as BTH (the manufacturer) and as Mazda (the brand) as BTH by this time had acquired the Mazda lamp brand and applied the name to valves. Note also the BVA logo but no BBC roundel placing the valve at the end of 1928 or later.
The lower end of the anode showing the grid and filament.
The balloon envelope is 42 mm in diameter, and excluding the b4 base pins is 100 mm tall.
Reference: 1043. Type P415 was first introduced in 1928.