Our Letterbox / Letter Plate springs are intended to be used with traditional style letterboxes with a flap that opens inwards. If your letterbox uses this type of spring you should be able to see the spring behind the flap. The spring slides over a steel rod that passes through two hinges. Please note our springs are not suitable for many modern styles of Letter Plate such as those found on UPVC doors. You may find our guide on how to fit a letter plate spring helpful.