I believe the Chicago Manual of Style says not to use the apostrophe with letters that aren't followed by periods. So, XYZs would be the correct plural while X.Y.Z.'s would the correct plural for that.
Sadly, I don't have my Chicago handy, so I can't look it up. :-(
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2/12/08 05:17 (UTC)Sadly, I don't have my Chicago handy, so I can't look it up. :-(