This is totally speculative, but since an "about face" is a drill command/move for reversing the direction you're facing, I'd say Dave picked it as a symbol for his solo effort being 180º from his efforts with Pink Floyd. Who knows. Ask Dave.

His hand, on the album cover, is not simulating 'playing guitar'; it's more like a hitchhiker's gesture. He's looking to his right, but gesturing to his left. 180º....hmmmmmm.....?

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Taking away from you for the greater good. (The Audacity of Socialism).

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