The design of the replacement flaps has changed slightly due to a change in manufacturing, and there are no longer instructions included to fit these flaps to older doors. If you get your flap and it appears to be about ¼" too short, don't worry. It is the right flap. All that is needed is to pull the magnet bar up to hit the bottom of the flap.
The old Pet Eze doors have a manually adjusted bar. The NEW Pet Safe doors have a floating magnet bar. This magnet bar is directly underneath the flap, and attract the magnets in the flap. The magnet bar is about ½" wide and usually sticks up about ⅛" higher than the rest of the frame. At first glance it may appear that the frame is all one piece, but the center part does move. Over time, that manually adjusted bar drops too low.
Grasp the bar on the ends with a pair of pliers and pull it upward, using a wiggling motion. It will adjust out to the top of the frame where you can make it fit your new flap tightly.
Actual pet door Flap Dimensions: