See this detail of original photo, slightly enhanced as to colour, contrast, etc. Sorry about size, something not right with usual "Postimage" facility.
[/url]
I think you are on the money, Oldfart.
Rear wheel does look like an open wheel. I thought maybe it was rear of clubman car, but car in photo is obviously rear-engined! "Rollbar" you mention is not rollbar, it is the side stringer of a small flight of steps leading from trackside to top of bank.
Car appears to be the Robertson Heron, but without rear section of body(at least). Driver is looking over his left shoulder, maybe saying "where's my tail gone? It was there a minute ago!"
Does this scenario ring any bells for anybody? Did he start without tail due to a practice shunt, or did it come off in the paddock? Don't think they necessarily had warm-up laps then??
Stu