The PE7B photocell from Mazda has Made in England etched into the glass as well as a paper label. The number 4388 is probably the serial number.
The underside of the composition base cap has the lettering BTH Co Ltd., it is possible, therefore, that this exhibit dates to the late 1920s when the Mazda brand was still made by BTH before the formation of AEI or just after while valves were still made at BTH Rugby.
The photosensitive layer is formed on the inside curve of the half cylinder.
The central wire is the anode and the cap at the top would have contained the material that was fired in the evacuated bulb to form the active cathode.
The wide glass tube envelope is 26 mm in diameter and, excluding the B4 base pins, is 94 mm tall.
Reference: Observation. Type PE7B was first introduced in 1929. See also1929 adverts.
