John ! You poor old thing ! I'd go mad without my books and papers...
It's from the big one - C7287 - WM20 25001 - WM20 42000 - Delivery started in August 1940. Mid-October 1940 would seem about right based on the Chilwell receipts.
Unfortunately, due to the time that he spends working in the fields, Signor Delcampo has clumsy fingers and has typed a '7' instead of a '6' ...Your number was C4666620.