Douglas had a few variations of sidecar chassis design, but none that I have seen quite like this. Particularly the triangulation under the rear cross tube; none of the other Douglas sidecars have that.
A typical 1930 and on sidecar chassis can be seen here:http://www.douglasmotorcycles.net/index.php?topic=1033.msg3420#msg3420
Though there were some other shapes of chassis frames. This has compression coils in the tubular towers at the rear, prior to 1930 they used leaf springs.
The wheel uses the standard Douglas hub, part #PC8690. Used from 1926 to 1938. Since the wheel shown has a beaded edge rim, it could be for a OC, 600EW, B29, A31, and their derivatives. Colonial models from 1929, and heavyweights from 1930 used the wired on tyres. Douglas used a special hub for cantilevered axles on the sidecar, but they may have used the standard cycle hub on heavy sidecar chassis where the axle was supported at both ends.