Long pieces are made of 50x5mm steel. Steel angles, 20x20x3mm, form hollow strengthening ribs for the long pieces, serving solely as reinforcement. The seat mounting studs are countersunk M8 10.9-grade bolts, welded from beneath. The front ones are covered by the angle iron, while the rear ones are visible positioned around the rear mounting hole, which is also countersunk to allow the rear seat bracket to rest over it. Washers around the mounting holes are used to raise the "rails" above the carpet, ensuring a steel-to-steel contact with the car body rather than a steel-to-carpet-to-steel connection.
I hope the photo taken before powder coating makes this clearer.
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