score:3
Because the LDS don't believe the Book of Mormon contradicts itself ("the Book of Mormon was the most correct of any book on earth", Joseph Smith), it must be referring to other materials which weren't mentioned in 2 Nephi 5:15.
Here are some examples of materials which were used in the temple of Solomon that Nephi could have been referring to, which were not mentioned in verse 15:
See also Bible Dictionary - Temple of Solomon