@spacekatia flat aluminium blank goes between the bottom one (underneath) and middle one (on top. The chamfered edges allow bending to precisely 90° along that line (need to over bend a bit to get 90°). Then the top one fits *over* the middle one (no metal in between) to allow folding the final tabs over the front, and hammering flat. Then the buck is unclamped slides back and is used to fold the bottom tab.
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