The Mazda AC/SG is and AC heated Screen Grid or tetrode amplifier for RF use. Our other AC/SG has a different construction with the original box screen grid.
The heater is the then UK standard of 4.0 Volts at 1.0 Amp. The original valves in 1929 had a metal top cap as can be seen in the advert. Also the advert shows an envelope with a cylindrical section at the base, just like this valve. The original S625 was made cylindrical as it was intended to be mounted with the screen mesh aligning with an external metal box that separated the input from the anode circuit. The shape of the envelope indicates that the missing top cap may have been of the metal screw top type.