Leo,
Looks like the correct item for a DT5/DT6. Technically the SW5/SW6 petrol tank would be the same except it would have provision to attach the hand change gate to the side of the petrol tanks (verses mounting off the frame as on the DT), and would only have one fuel tap outlet (verses two as on the DT). However I have my doubts as to how many SWs were really supplied that way, and suspect that SW models used the same tank as the DT.
I think you will find it is silver/aluminum paint and not nickle plating. They were not plated originally. A tracing of the paneling, lining, and location would be helpful for my SW5, as it does appear to be the original factory finish.
DTs were supplied from mid(?) 1928 to 1932, and the SW model from 1930-32.
Doug