The tanks had panels in cellulose paint over chrome plating - the '31 and '32 models had a tartan surround to the side panels. In 1930, 3 models were available - the S5 (500cc) and S6 (600cc) were the sports models - they had normal footrests, a single spring front fork, and no rear carrier. The T6 (600cc) was the touring model and had two springs on the front forks, footboards (instead of footrests) and a rear carrier. Your bike also has the three speed gearbox which was standard in 1930 - the 4 speed box was later offered as an option. In 1931 the S5 became the C31, and the S6 became the D31, with the T6 becoming the E31. Then in 1932, Douglas decided to give their bikes dogs names, with the S6 becoming the Greyhound, the T6 was the Ayredale, and a larger 750cc version (for sidecar work) was named the Mastif. Despite the change in model names or numbers, the bikes remained basically the same.
Getting back to your bike, it's good to see it still has it's original Powell and Hanmer headlight and a period Cowey speedometer.