I have both of these (venting and none venting). The venting type (UK) does so by holes in the cap mechanism (if I remember right, I have pictures of both to show the difference but can't show them at the moment). The non venting (N/A) is part of the sealed N/A system to prevent gas fumes entering the atmosphere. It vents through the emissions/fuel separator system which is why people who remove emissions can develop vacuum lock if not done correctly.
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