Character mistake: The first time that Diana mentions the mysterious civilization that produced the 'treasure', she mentions that "They survived in isolation until the 1500's." It's a plural, 1500s, no apostrophe.
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Suggested correction: Both versions are acceptable depending what style guide someone chooses to follow. Anyone saying different is just stating an opinion (well, until all style guides decide to follow one rule). It's similar to plural letters, as in "mind your p's and q's."
