The first machine is an E29, first year for the saddle tank. The engine was pretty much the same as the previous year E/F28, except for the Enots oil sight glass on the timing chest, rather than the built in Douglas sight glass (which really could not be viewed all that well when riding.) The F29 was similar except it had foot-rests and sports (slightly dropped) handlebars, and I believe aluminum rather than cast iron pistons (the 1928 models were advertised so.) An electric model, but the BTH four-pole dyno has been removed from the timing chest and a blank-off plate fitted in its place.
The second is an S6 or derivative, of 1930-32 vintage. I can not tell the various years apart, as they changed so little! As with the E29, acetylene lighting has been fitted and the BTH dyno removed.
The last unfortunately is from a newspaper or magazine halftone picture, so the detail is not as good. But good enough to tell it is a 1934 due to the lack of airbox on the timing cover. No sign of hand-shift, so the foot-change gearbox. It has the Amal carburetor fitted (same as the S6) rather than the Douglas barrel carb (either could be fitted.) 1935 OW/OW1 reverted to an airbox, and while this on has some sort of box over the carb, I think it is just a shield. But since it is said it was a 600, then it was the OW model. I have a personal interest in the OW/OW1. It is estimated about twenty five OW/OW1s were made over the two years, of which eight are known to have survived. They have a very good turn of speed, using up many of the internal parts of the Dirt Track machines!
[edit 01Feb15. Change third machine from 1935 to 1934 due to lack of airbox. At time it was thought only 500cc machines in 1935 might have gotten the airbox, but since 2005 a 600cc OW1 from 1935 has turned up with airbox. So lack of airbox = 1934; precence of airbox = 1935. -Doug]