This example of the AC/P has the classic envelope and is of later production than AC/P with the balloon envelope. It was probably made much later than 1930 and produced for replacement purposes, rather than for new radios. The printed identification says Mazda but the genuine Mazda valves are AC/P not ACP. There was nothing else visible on the glass to alter the view that it was a typo on a Mazda valve.
As an output valve the power available was up to 640 mW.
When processing this image of the helical wound heater it became clear that there was some etched lettering that could not be seen with the naked eye - or even under a glass. Some extra processing was required.
Bringing up the yellow content of the image revealed an etched lozenge and a clear R27. There was space before the R and after the 7. More processing required.
The R27A has to be a UK Navy pre-CV valve NR27A. Looking at the ML4 etched lozenge indicated a close match in style. So this AC/P looks to have been made as a NR27A, possibly by M-OV, and later re-branded as the Mazda AC/P.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 95 mm tall.
References: Data-sheet, 1040 & 1043. Type AC/P was first introduced in 1929.