Hidden inside the Sport Band slot, we spot a mysterious cover—a door of sorts.

What is the door for? Where does it lead? We may never know...but it looks like the reported diagnostic port.

Taking a closer look at the side of the Apple Watch, we find the model number: A1553.

The Apple Watch comes in two different model numbers: A1553 for the 38 mm case, and A1554 for the 42 mm case.