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in the one you have shown the 9v+ connection is a sprung metal clip which is in the blanked-off off-centre bit next to the socket. it's my guess that a big tensioned metal clip like that helps keep a tight grip on the jack so preventing possible shorting or the jack getting knocked out too easily.
but they aren't always off-centre.
'be a good animal, true to your instincts' (d.h.lawrence).