The AC/TP is what it says; an AC mains Triode Pentode. The unusual feature is the B9 base.
Unlike the AC/TH1 the Mazda AC/TP triode-pentode frequency changer was specifically designed to accommodate a limited amount of frequency pulling, but it does provide a reasonable degree of isolation between the signal input grid and the oscillator, necessary to prevent radiation of the oscillations from the set's aerial.
The AC/TP was thus a good valve for a cheap, medium-wave, domestic superhet which would only be used on strong local stations and must be as forgiving as possible when tuned in a slapdash manner.
Millions of these valves were made and they are as tough as old boots, whereas normally the frequency changer is the Achilles' heel of a radio. Introduced around 1935, the AC/TP was obsolescent (though still in production) by 1945.
The three exhibits we have show specimens covering this period. Earlier specimens, such as this, tend to have green (rather than black) markings.
The triode would be used for the local oscillator and the pentode would act as the mixer.
The classic envelope is 44 mm in diameter and, excluding the B9 base pins, is 105 mm tall.
References: Datasheet, 1040 & 1043. Type AC/TP was first introduced in 1934.