That does not look like a bona-fide 410 petrol cap, indeed it looks much more like a radiator cap. I attach (I hope) some photos of the genuine article which consists of a circular spring loaded disc with a rubber facing that bears on the smooth end of the filler pipe. It is a very effective and simple piece of work - opening the flap automatically opens the cap and the key to the flap cannot fall out on the road, nor can one drive off with the flap open.
Roger
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