Claggart is a unusual name indeed, it stands out from the rest of the story. The name is pretty cacophonous and well it's never used before and probably will never be given the association given by Melville

http://goo.gl/oxGvuF(need Jstor access)

Such an ambivalent phrase. The narrator seems to be claiming that this is a "moment" unparalleled in its significance. Yet this significance is "undetermined"; it remains unclear exactly how the moment is significant.