"We find a dividend coupon for 5d under the lid of an Oxo Cube tin."
Old Oxo Cube tine
Creative Commons AttributionOld Oxo Cube tin - Credit: practicalalowl

In this story the boys raid the kitchen for money-off vouchers from tins and packets, and dividend cards from Black & Greens tea to exchange half-price to an unscrupulous general dealer - a common practice at the time.