Used to be really common for flex plates to have six converter bolt holes. But they were three on a "large" bolt circle, and three on a smaller bolt circle. Only three would be used in a given application, depending on the size of the torque converter. I don't have a "good" photo of that, but I've got a bad one. (Better photos on another computer...)
Buick "Big-Block" (400, 430, 455) flex plate showing weight reduction holes, balance holes, and--if you look carefully--two sets of three torque converter bolt holes, one set of which has witness marks from being used.
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The heavy-duty flexplates--for 4L80E, and on TH400 converters behind some very few high-powered engines--have six holes on the same bolt circle.