Throughout the Susan series, Bill Carmichael is renowned for being mechanically minded and good with his hands. He makes a special flap in the kitchen door to allow Susan's cat Chang to come and go at will and often works with Meccano models. However, in the last two books of the series, he takes an interest in philately and starts to build a stamp collection. A friend of his from school called Stobbs helps him. The stamp collecting ties in nicely with the fact that Susan and Midge are pursued all over Venice by desperate stamp traders in search of the Shilling Deep Green. It is Bill and Stobbs who confirm the value of the stamp by telegram.