A word of warning on Rover catchers. I have learnt the hard way that there were actually about 4 or 5 different rear catchers made by Rover in the 70s for different models. I purchased a blue one, but it was far too big. I've been told it must be for a Challenger. I wanted one for the Rover Princess 18.
I then purchased a black catcher which came off an old Rover Princess, but it won't fit my mower either. Perhaps mine was a later version and they modified the design? All I know is that the catcher won't fit unless I remove the rear flap, and then of course there is a big gap in the top. The rear flap on mine looks original, but it is not flat. It has a 90 degree bend on one side, which is about 6 cm long by 5cm deep. This gets in the way of both catchers I've tried on it so far.
So I am at a loss to know what the original catcher could have looked like. It seems to me it must have had a wider opening than any of the ones shown above, or you had to remove the flap entirely, in which case the catcher had a flat opening on the face, with no opening on top.