The 6P15 is the Type designation for Mazda's version of the EL84 audio output pentode.
When used as a pair in an ultra-linear push-pull configuration with 43% distributed loading the output could reach 10 W with 0.9 % total harmonic distortion.
With ultra-linear operation the screen grids were connected to taps in the in the output transformer primary winding. The valve operating conditions so produced were intermediate between a triode and a pentode. The operation of the distributed loading was to introduce negative feedback directly within the output stage.
This exhibit dates to 1961 and has been taken from a working Murphy radiogram type A674SR that uses Simplex Stereo amplification.
Using the μ-Tracer the above curves were produced. Even after 50+ years it was still at 90% of the nominal design values.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 67 mm tall.
References: Datasheet, 3002 & 1040. Type 6P15 was first introduced in 1958.