I've always put the thumb-knobs on the ejection port side.

As a right-hander, I sling my rifle on my right shoulder. When being carried slung the knobs won't rub/loosen. Makes sense to me.

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Apparently Finn Aagaard thought the same way, likely for the same reason.

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“Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away” Antoine de Saint-Exupery