Originally Posted by oppozit
It's a Watsonian "Flight" and an early one because of the "fin". The fin was dropped on later models for a standard Watsonian luggage rack (two aluminium castings bolted to the shell.) There should be a makers plate nailed to the woodwork under the seat squab with a serial number and date of manufacture. It would've originally had a 10/12" scooter wheel mounted to the brakehub.
Ya ...Watsonian Frame # "R604" ....good call. My friend just sent me a photo of the frame stamp. I have another friend with an old "Pop Deyer (?)" sidecar which has a similar fin: