Nephi states:
"I did read many things unto [my people] which were engraven upon the plates of brass." . . . but that I might more fully persuade them to believe in the Lord their Redeemer I did read unto them that which was written by the prophet Isaiah." (1 Nephi 19:22-23)
Because the Book of Mormon gives direct evidence that the plates of brass contained Isaiah chapters 2-14, 28-29, 40, 43, 48-53, perhaps 54, and 55:1-2, the Book of Mormon student is allowed to postulate not only concerning the date in which the book of Isaiah took its final form, but the date of it's incorporation into and the manner of compilation of the plates of brass.
According to John Welch, if the plates of brass were not made and inscribed until around 620-610 B.C., this would allow time for possible collecting, editing, redacting, or supplementing to have been done to the writings of Isaiah after his death, around 700 B.C., and for that work to have already entered the standard version of the biblical text before the Isaiah texts were written on the plates of brass.. . .
Although earlier dates for the making of the plates of brass are possible, it makes sense to view them as a royal record compiled and inscribed around 620-610 B.C. by King Josiah, who reigned from 640-609 B.C. The plates contained the book of Deuteronomy (1 Nephi 5:10), and that scroll was most likely the book of the law that was not discovered by Josiah until 625 B.C. That discovery made Josiah and others in Jerusalem acutely aware of the fact that books of scripture could get lost, which would have motivated them to do everything in their power to create a permanent archive and a durable copy of their most sacred records to prevent any loss of scripture from happening again. Moreover, Deuteronomy 17 requires the king to have a copy of the law and to read in it all the days of his life. The rediscovery of the forgotten book of Deuteronomy that contained this particular scripture could have prompted Josiah to see record keeping as a royal function and to make records that would not wear out or become illegible through extensive use. In addition, the plates of brass were in Laban's custody in a treasury. The text simply says, "Laban hath the record" (1 Nephi 3:3), not that he necessarily owned them. Because he commanded a garrison of fifty soldiers inside the walls of Jerusalem, Laban may have been the captain of the king's guard or a high-ranking military officer. His treasury could have held public as well as personal records. While the plates of brass contained important genealogies, it is not likely that records of this quality would have been "family records" alone. Perhaps the genealogies served several royal purposes, such as settling disputes over marriage, inheritance, property, or other legal claims based on family status. Finally, dating the plates of brass to the end of the seventh century is consistent with the fact that they included information down to the commencement of the reign of Zedekiah and many prophecies of Jeremiah, who began to prophesy in 628 B.C.
Of course, other possible dates and scenarios can be imagined. Perhaps the plates of brass were a sacred record that had been kept up to date all along by prophets who preceded Lehi. Perhaps Laban had confiscated this book making it property of the state, when one of those prophets was put to death for prophesying against Jerusalem and the king. [John W. Welch, "Authorship of the Book of Isaiah," in Isaiah in the Book of Mormon, pp. 430-432] [See the commentary on 1 Nephi 3:3]