The DC/SG from MAzda was introduced in 1931 and re-designed in 1933. The AC/SG, designed with the UK standard 4 V 1.0 A AC heater was introduced in 1929 and was also upgraded over time. Although not quite the same electrically the AC and DC versions are closer than the earlier and later versions of the same valves.
The DC/SG is a 'straight' RF tetrode or screen grid valve.
The top cap is Bakelite with a screw connection, the metal 'top hat' style arrived in the mid 1930's.
The screen grid is a rectangular box of close wound wire with caps top and bottom. This original format of a complete screen gave way to the simple wire wound grid later in the 1930's as greater understanding of the technology developed.
The balloon envelope is 45 mm in diameter, and excluding the B5 base pins is 113 mm tall.
Reference: 1043. Type DC/SG was first introduced in 1931. See also1931 adverts.