you can see the very basic design from the 2nd photo, the curve at the end really doesn't affect the frame, it just sits slightly high at the back... makes no difference.
This type of frame design was great because nothing came into the tub itself allowing you to fully utilize space, the side rails are more than strong enough
It is also very quick and easy to swap it out (normally i run around with the hard lid on) but generally a 2 man job due to awkwardness of the frame size