I know I have read about this in a book but I cannot find it.
I thought it was 13 Steps or PMM but no joy unless I have overlooked it.
In the diagram I remember seeing it had a box with a curtain on the front and one of the things you could do was make a ball hop in to the cup.
If someone can confirm which book this is in it would be really useful and narrow down my search somewhat.
Unless I just imagined it.

