The P615 is battery audio output triode. In 1928 BTH had the Mazda brand and the formation of AEI had not taken place. This places this exhibit to 1928 as the P615 box shows. The slightly earlier LF407 is packaged as a BTH valve with a small Mazda logo. Here the Mazda brand dominates with the makers name as a reduced size logo. Also the nickel filament was advertised in 1928.
The dome carries the etched lettering. There is just sufficient detail to confirm that this is a P615 (bottom line) with a 6 V filament.
The gettering obscures the electrodes but the shape of the anode box can be seen. It is much wider that the front to back dimension and appears quite tall.
The pinch and electrodes. The M shaped filament is oxide coated.
The balloon envelope is 43 mm in diameter, and excluding the B4 base pins is 111 mm tall.
Reference: 1043. Type P615 was first introduced in 1928.