The first or first few models of 104 had downward seats. And possibly a few other craft at the time. The why for it is because they couldn't ensure a safe punchout at supersonic speeds over the high T tail of the 104. But it obviously meant that the pilot needed quite a distance under the keel before ejection was safe. Rolling to the side was an option of course but that's hardly a good thing to do during takeoff and the initial portion of the climb when speed is low. It was the best they could do at the time.