"Nez takes us along to the Roll a Penny where he's figured out an easy way to win some money back."
The roll a penny stall had numbered squares. The idea was to land a penny inside the square without touching one of the border lines, and you would win pennies corresponding to that number (rarely more than fourpence). The stall was surrounded by wire mesh, but small fingers could sometimes poke in to manipulate a penny onto one of the nearest line of squares. Woe betide you if the stall-holder caught you doing it, which is what happens to Nez in the story.