The origin of the brass plates

We know from Biblical history that the Israelite kingdom was divided in two. The northern kingdom consisted of 10 tribes while the southern kingdom was made up primarily of the tribe of Judah and Benjamin. Since the tribe of Judah was larger, these people were referred to as Jews. The tribe of Ephraim, Joseph's son, was the main tribe in the northern kingdom. So when the Old Testament talks about Judah and Ephraim, it often talks about the two kingdoms.

A close reading of the Book of Mormon reveals quite strong indications that the brass plates originated from the Israelite northern kingdom (the 10 tribes) as a Josephite/Ephraimite version of the Old Testament, as opposed to the Jewish version we have today.

• The fact that Laban was a descendent of Joseph is given as a reason for him having the brass plates. 1st Nephi 5:16:

And Laban also was a descendant of Joseph, wherefore he and his fathers had kept the records.

• They contained a genealogy of Lehi's forefathers, who was also a descendent of Joseph (See 1st Nephi 5:14)
• The brass plates contain the writings of prophets who are unknown in our common Old Testament originating from the Jews: In particular, Zenos, Zenock, Ezias and Neum are mentioned (see 1st Nephi 19:10 and Helaman 8:20)
• At least Zenos and Zenock were from the tribe of Joseph. 3rd Nephi 10:16:

Yea, the prophet Zenos did testify of these things, and also Zenock spake concerning these things, because they testified particularly concerning us, who are the remnant of their seed.

• The brass plates contain extra-biblical writings of Joseph. The most prominent example is 2nd Nephi 3, but also Alma 46:24 contains a quote from Jacob/Israel to his son, Joseph, which we don't find in the Old Testament
• Zenos did not seem to reside in Jerusalem. 1st Nephi 19:12-13:

An all these things must surely come, saith the prophet Zenos... And as for those who are at Jerusalem, saith the prophet... 

It should come as no surprise that the Book of Mormon prophets are very concerned with "a remnant of their seed". They know that they are descendants of Joseph and that he received special promises about his posterity. They had the brass plates which contained those promises, more so than our current Old Testament.