Ian, From the details you have given, the engine is a T35 (Mk1) number 647 but the frame number quoted is incorrect. 6922 would make it very late in the Mk3 series. The correct number should be either T35/S/6922/3 for a Mk3 or T35/S/6922/S for a Mk3 sports. Obviously the frame has been changed at some time - this is not unusual as the early T35's were known for their frame breakages. From the pictures posted, it has been built as a standard Mk3 with a Mk1 engine/gearbox. If you later join the London Douglas Motorcycle Club, the membership secretary will probably be able to help with date of despatch of the original bike and/or the replacement frame, and even the dealer it was despatched to (which may help in discovering the original registration mark).
Good luck with the restoration,