Joe,
Any idea if your Gold Rhino is one of the original runs or a re-issue?
If a re-issue, the newer coverstock could be enough of a different formulation that it is now "stronger". (Just a stab in the dark, as from your description, I wonder if it isn't the cover that is so muchstronger.)
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Just like hand grenades and horse shoes, in bowling you only have to get close ...
CharlesT