Prior to 1968 they were.
The counterfeit detector pens, afaik, indicate the presence of wood fiber, something present in notebook paper, but not U.S. currency, comprised of cotton and linen. So the older currency should pass this test.
The counterfeit detector pens, afaik, indicate the presence of wood fiber, something present in notebook paper
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I think you’re right about that ,, or at least they detect acid which is common in cheap paper