I don't want to sound sacrilegious on the Douglas forum, but by 1925-6 belt drive was well on the way out for British motorcycles and Douglas should have discarded it years earlier. Of course there were cost issues, and some very conservative customers who liked the stories about smooth flat twins and smooth belt drive compared with harsh and dirty chains. By the mid 1920s, chains were high quality and not at all harsh, particularly if lubricated and adjusted correctly. If the bike was up-market enough to have a cush drive somewhere, the superiority of chains was obvious to all. That said, Douglas sold thousands of "old fashioned" belts drive bikes so there was some business merit in there somewhere. Apparently the last of the belt drive bikes were quite a hard sell...
Leon