In the first year or two of the WRX in the US, there was a change to the calipers and brake pads. If you have a WRX (or Impreza) made at this time and are buying pads, it’s good to double check by actually going and looking at the brakes on the car. Here are some pictures that show the difference.
First are the front pads. On the left is the older, 98-02 shape, on the right, the newer, 03+ shape. The 11+ shape D1539 is also compatible with D929. The easiest way to tell the difference here is by looking at the caliper and bracket itself.
Here is the rear pad, shape D770, found on the 98-02
This is the newer, 03-07 rear pad shape