if this is a plate that doubles as a nut, as other Answerer's info. seems to suggest, or an ordinary nut used to secure a plate, which was my initial reaction.

Either way, I think "plate nut" says it !!!!

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Note added at 26 mins (2006-11-16 15:07:41 GMT)
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Not to be confused with those people who collect china with pictures of horses, elephants, strawberries, the Royal Family, etc. on it, of course.

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Note added at 1 hr (2006-11-16 15:59:47 GMT)
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Actually, I might rather have expected a plate acting as a nut (either with a threaded hole or with a nut welded over a hole) to be called a "platine écrou". There are three ghits for this, concerning rectangular and diamond-shaped "platines écrou". Not that an "écrou platine" might not be the same thing, especially as seen on the other side of the looking glass.