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Useful
I think the word you're looking for is facetious :)
Jurek gave the non-facetious version of the same answer - it's a proprietary design, there is no standard nut which looks like that. The spherical part seats in a matching recess in the rail clamp presumably? The general term is a spherical seating nut, but they usually come with matching washers and have a hex portion for a wrench opposite the washer part.
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gbj_tester
Thrasher
Useful.