I think Chris basically covered this in post 3?

A theory of course but if the early group 4 cars were round arch then this required a bespoke mould to make, later in the timeline it will have been easier to pull a Stradale rear canopy from the production line & convert it to GP4 with the simple addition of a wheel spat/extension.
The section of the arch on the centre tub will always have been rivetted on to a standard Stradale steel centre tub