Hi Manfred,
Prewar, Douglas offered their machines modified to the customers requirements (presumably, this incurred an extra charge) and could have included change of colour, and possibly the oversize wheel rims - so your bike may have been built to special order by the factory, or more likely, it has been modified in the intervening years. Having looked at the photo you have just posted, someone has made some changes from standard. The gearchange gate on the side of the tank is not standard - as the gate is set at a different angle. Originally the rear fixing would have been used for the kneegrip rubber, and the gate would have been mounted on the forward pair of bosses. Also, the gate gives much more movement than the original one (and is for a 4 speed gearbox) - so maybe there has been a change of gearbox (although Douglas did offer the option of their own 4 speed box).
Eddie.