A little fountain pen mystery is afoot. I somehow have an extra inner cap for a Platinum pen. All of my Platinum pens with that style of inner cap have theirs in place and intact. I don't recall any of the ones I bought coming with an extra, so I have no idea what it's from or what it goes to, but it's a 100% match for a Platinum inner slip and seal cap from the Preppy/Prefounte (and I think Plaisir) style pens.
It must be an extra, but from what?

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