Complex sentence: In 1904, the Mint set records for production of double eagles: 6,256,699 at Philadelphia and 5,134,175 at San Francisco--highs for the series for those mints.
Split into simpler sentences: In 1904, the Mint set records for production of double eagles, striking 6,256,699 at Philadelphia and 5,134,175 at San Francisco. Both set highs for the series for those mints.