This is the hinge area on the left side of one of the seats. Left to right, the three bolts are A) pivot for the seat adjustment slider, B) the hinge bolt, and C) the seat back angle adjustment stop bolt. The latter is a commercial, UNC Grade 8, 5/16" X 1 1/4" bolt. The four such stop bolts (two for each seat) will be the only non-aviation specific fasteners in the whole airplane, and they're not fastening anything here, just taking compression loads.
