I think your right CH that the 90% are an alloy. The Kennedy halves are big coins and could easily be clads, although modern quarters are also clads so I don't know how thin a coin would have to be before it wouldn't be easy to make it a clad, maybe there is not limit (isn't a modern dime a clad?). Try cutting a 90% quarter or dime in half and look at it under a jeweler's loop. You should be able to see if its a clad or not.
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