There is a drilling in the side of the plug in which you can put a size 3 Allen key or a Torx key T10 (usually found in your BMW toolkit) in order to fit and remove it.
See tools.
For spare parts see filler pipe (part number 7659578) and o-ring (part number 1340901).
Also referred to as oil filler cap.
Chrome cap II was created for covering the gear oil filler...
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This oil filler plug with Allen tool looks absolutely great...