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Detail of the triangular cleat on the bottom of the shoe.

Picture 2 of 3.

This shoe appears to be an early prototype, as the cleat differs from those actually worn in flight (scroll down to the tenth and eleventh photo for views of the bottom of the shoe and cleat).

In particular, the flight shoe allows the cleat to be repositioned anywhere along a track from the ball to the arch of the foot, and the triangular cleat has a wingnut. This is presumably to allow the cleat to be positioned in the track, and I speculate that it may allow the cleat to be removed (e.g., in case it were to get stuck in the grid).