Euan,
Your photo shows one of the very few curtains I have seen which are made with the steel rod instead of the flat bar construction. I note there is also the triangle access flap like mine.
I suspect my originals would have had a bit more 'slack' in them too, but the need for them to be disassembled for examination of construction method has now reduced them to a pile of rags. Very old and stinky rags at that. Mind, the the canvas was in dreadful state, so I don't feel any remorse with not being able to keep them as they were.
The truly odd thing is that the rod frames were different to each other. One had the angle of windscreen slope, other was a simple rectangle. The rectangle one had a longer portion of canvas at front to accomodate the shorter frame, and still allow it to fit sloping windscreen. very strange!
I still have the frames around. They will be kept for "Justin"