Originally Posted by DeMac
The grey double layer over the wells, as there is material missing and no floor-to-wall joint. Like a door or window header, in buildings. Idk, I'm just a guesser.
My bus also has extra reinforcing material between the ribs above the wheel wells. My guess is also that this is to compensate for the large cutout in the chair rail beam that is needed to accommodate the wheel wells. Every place in my bus where the chair rail is "broken" in some way (wheel wells, the side exit door, and the cutout for the fuel pipe) there is extra structure to compensate for this.
OP, what you've posted looks kinda homegrown, though, something that seems to happen from time to time with buses in long service. My bus, for example, had a rib replaced at some point in its past with additional cross-member support welded to the underside.
You should be able to access all of this from the in
side by cutting the rest of the interior panels off (but not
the chair rail at the bottom!). The interior side panels don't do anything but prevent the kids from eating the insulation (I think that's all the ceiling panels do as well but that's more controversial).