Thread sizes below 1/4 confusing

Its a speciality nut I am looking for. It has a Hex Nut with a free spinning washer with rubber backing. It's 10 - 24 with a 5/8 washer. It holds on mouldings on an imperial and the rubber stops water intrusion into cab, trunk etc. www.clipsandfastners.com have this type but most of them metric sizes.
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