The boxing kangaroo is an Australian national symbol often identified with the nation's sporting teams. It comes from the actual behavior of kangaroos, who appear to "box" as a defensive move.
Hoppy breaks the fourth wall at the end of the short by saying a trademark phrase of comic Jimmy Durante, on whom the dock cat's voice is modeled after.
Sylvester very subtly breaks the fourth wall, with a fleeting but knowing expression.
The Acme corporation has expanded to the warehouse industry in this short.