Colin,
Most SW5s are converted DT5 frames, though some have been skillfully done such that it is hard to tell what is original. Folk add a second footrest, generally a stub welded directly to the frame tube rather than the factory method of a brazed lug. However the most telling difference is the DT has the footrest(s) on the engine bearers about five-six inches forward of the rear down tubes. The SW5 would have the footrest right back near the down tubes, perhaps two inches away along the engine bearers. The footrest on the SW5 are further back. Also the left hand footrest would incorporate a sleeve and stop for a brake pedal.
Then add a front brake anchorage, brakes at both ends, and a flywheel clutch and you have an SW5. The SW5 also had the hand shift gate attached to the petrol tank rather than to the frame. I only know of one machine with that feature. However I believe that the factory themselves dropped that and at some point both models used the DT style gear change quadrant attached to the frame.
For an extra 10 quid the factory would provide a specially tuned engine by Rex Judd and Freddie Dixon that produced 32hp rather than the standard 27hp. However the 'Red Devils' were the creation of tuner Bert Dixon, and were alleged to produce 34.5hp.
The smaller tank could be a Red Devil tank, or just a smaller tank someone made up years ago. Verification would be quite difficult.
-Doug
