I've never heard of a Z class flare before, and I can't find any mention of such a thing on the NASA web site. Beyond X9 they just go to X10, instead of (say) Y1.
Someone recently added "Z class" to the table on the Wikipedia Solar Flare page without any explanation. If, as that table says, it means > 10^-3 W/m^2, then we've already had several: the X17 flare on Sep 7 2005 would be Z1.7, for example.
Where does the figure of "a least 40 times as large as March's display" in the article come from? The March 29 flare was X1, so 40 times that is not a particularly interesting number. Or did you just misunderstand this statement on the Wikipedia page: "the extreme event in 1859 is theorised to have been well over X40 so a Z class designation is possible"?