I'll bet they steamed those covers to shrink fit them onto the metal seat pan. There is no way you could ever take mine off without destroying it. Either that or that plastic material was very much more flexible then than it is now. I have one small separation (an inch or less) along the side, where the bike was dropped on it's side somewhere along the line. Other than that, it's in really good condition.
BTW, when you get to the point you need an exhaust, if you haven't found one, let me know.
Mike Sal