Interesting that you should ask. Just finished a little internet research which included cycleback.com's discussion of counterfeits and authenticity of early baseball cards. Quoting Cycleback, " If paper stock fluoresces very bright as just described, it almost certainly was made after the mid 1940s. It is important to note that not all modern papers will fluoresce this way as optical brighteners are not added to all modern paper..."
Happy collecting,
Larry
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