Just a guess. If the pieces are outside [under] the belly pan they may be intended to hold up something inside the pan like an add on gray water tank, a '66 had none originally. The forward edge of the black water tank should be in that area. The tank sits in a sheet metal pan bolted up through the floor. Its common for the pan to rust out and let the black tank drop down, something you'd probably replace on a '66 anyway. The 2x4s must be attached to a crossmember I doubt the bellypan aluminum could support them for any lenght of time. If the bolts are attached to the main frame rails the frame has been drilled, a real no-no. The bore holes would have to be welded/reinforced. Just the normal kind of things you find on a 44 yr old trailer, and a '66 T W is darned hard to fine in any condition.