The Mazda AC/SG is and AC heated Screen Grid or tetrode amplifier for RF use. It was made in both clear and metalised versions. The internal structure can be seen on the clear version.
The heater is the then UK standard of 4.0 Volts at 1.0 Amp. The top-hat top cap arrived in the mid 1930's. The first versions had screw metal caps and later production the black moulded screw top caps.
The balloon envelope is 45 mm in diameter, and excluding the B5 base pins is 105 mm tall.
References: 1040 & 1043. Type AC/SG was first introduced in 1929.