Found a wiring diagrams for 1950 that shows a negative earth for the Mark 4. I can scan and email this. I thought the post-war Douglases with dynamo started out as positive earth, then perhaps switched to negative earth. But I have come across some info that states it is the other way around. According to the Lucas electrical section of Modern Motorcycle Mechanics (1953 edition) it states in early 1952 a change over to positive earth was made from negative earth. But it also says from 1949 on the longer 60 watt dyno was available. Douglas was still using the short dyno on the Mark 4, according to Lucas' own data sheet (the one I think the service manual refers to.) So did they carry on with negative earth? It is still an open question.
-Doug