Historically speaking, if the Brass Plates contained an early version of the Old Testament, which was certainly in Hebrew. From the historical perspective, it makes more sense for the Brass Plates to have been in Hebrew, which would indicate that "these plates" in Egyptian would be the Book of Mormon plates.
First the structure is really throwing me for a loop, especially the placement of "if" and the use of "which". Then the placement of a period and starting a new sentence. It just does not parse for me easily.
So I parse it as:
The brass plates, if containing some early version of the Old Testament, was (referring to some early version of the Old Testament contained on the brass plates) certainly in Hebrew.
Simplified asserting that: the brass plates were certainly in Hebrew if they contain an early Old Testament.
Did I parse this incorrectly?
He did unmangle the sentence later. The new version is still cumbersome. But am I the only one who thinks the original was a thick mess, and it parses as I suggest?