FYI: The disappearing bat is the rarer one.
There is a theory regarding it. If you look at the one clearly showing a bat, the magenta in the skin tones is clearly richer and more heavily saturated. Those hues are also evident in the bat, so if there wasn't enough magenta ('red') when the sheets were printed, the bat would fade into the sunset, so to speak, as it does on the 'no bat' card.
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