tck,
The postwar series started with the T35 (it never assumed the title 'Mk1' until later versions appeared). Concurrent with the T35, Mr Douglas was experimenting with other models, including one with shaft drive - but nothing became of them. Somehow, a Mark 2 never reached the marketplace - the T35 being replaced by the Mark 3 and it's sportier sibling - the Mark 3 Sports, this being given another 3 BHP by fitting the sports cam (stamped with an 'S')(the standard cam was stamped 'T' for touring). Both the touring and sports models continued into 1950 being labelled Mark 4 and Mark 4 Sports. Later in 1950, the sports version was dropped when the bikes developed into the Mark 5's. In the period spanning late Mark 3 production to the early Mark 5's, Douglas were also producing the even sportier 80 & 90Plus models. Early development of the Plus series was not without it's problems and on a Mark 3 Sports, Don Chapman beat the 'Pluses' to win the 1950 350 Clubman's race at Silverstone.
Hope this helps,
Regards,
Eddie.