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
While the older notes lack the more modern anti-counterfeiting measures present in the new notes the pen shouldn’t have been necessary, really. There are fine red and blue silk fibers embedded in the bill, for example, and the engraving should be sharp, etc. Afaik, though, the older notes should pass the pen too.