The PP3521 from Mazda was specifically designed for good quality audio output when used with the limited HT available from DC mains. Normally a pentode was used in these sets for the greater sensitivity but would be more likely to offer a degraded sound under unfavourable circumstances.
During the 1930s Mazda indirectly heated audio valves were well regarded and the PP3521 could be seen as Mazda's version of an indirectly heated PX4.
Thanks to Mike Edwards for sending these images. Mike comments: I have recently restored a KB 381 and discovered to my surprise it was fitted with a PP3521 triode in place of the normal pentode output valve which was listed by KB as 7D3 or 40PPA.
The control grid has a cylindrical heat sink fitted.
The Mazda name and Type designation etched into the dome.
The classic envelope is 54 mm in diameter, and excluding the base pins is 111 mm tall.
References: Datasheet, 1040 & 1043. Type PP3521 was first introduced in 1936.