It seems the chain drive may be correct, there are models earlier than 1918 thus equipped...Ian

email (option): ian@wright52.plus.com

Ian,
Chain drive is a model H , I have a 1915 model H. It cost an extra £4.
Same engine front end etc.
The main difference is the backstay, chainstay. Rear hub is the same however it is fitted with the sprocket rather than the belt rim. Same with the gearbox.

It also has a cush drive rather than just an engine sprocket.