I have a 3300 and know exactly what you're talking about. Did a search and can't find anything similar either. There are 2 of these plastic brackets mounted to the boat; the stainless loops on the back edge of the seat slide in the narrow slots of the mounts when you put the seat in oriented at the correct angle, then when you flatten the seat the stainless loops rotate and they're unable to slide back out of the bracket. The flat parts of the loops are clocked away from the mounts' slots when the seat bottom is flat, so the loops are physically unable to fit through the slots at that angle.
One of my plastic mounts is broken on top, but the bottom part is still intact and holds the seat well enough that I haven't had a problem. I thought up a way to create new ones with some aluminum plate and a few pieces of aluminum L channel (see pic of my crude drawings lol). All you need is something that lets the loops through and sit on top for support. When mine break, I'll probably build something like this, or weld something up out of small tube that has a slot of similar size and orientation.
2003 3300 SE