Is your door or wall too thick for your cat flap? This PetSafe tunnel extension 310, with an extra depth of 48 mm (1 7/8"), is a great way to extend your cat flap for particularly thick doors or walls. Add a single or multiple tunnel extensions for your cat's ease. This PetSafe tunnel extension was formerly known as the original PetSafe tunnel extension. It is compatible with the PetSafe Deluxe manual 4-way locking cat flap (300 series), the PetSafe Deluxe magnetic cat flap (400 series), and the PetSafe Deluxe infra-red cat flap (500 series).