I "think", the screws on the "floor", closest to the trap door hold those chrome pcs down, the other two screws, that go all the through the "floor" go in to two copper colored "spring" clips, those clips slide into two brackets welded onto the chassy.
Now for the front edge, there are two holes in the underside of the floor, and they look like they should be mounting to the hinge plates of the (moving)fold down part.
Man this is tough!
Bottom line? my "floor", or flat pc just slides (and bounces) around back there.