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Isaiah 9:5-6 LXX:
⁵For unto us a Child is born, unto us a Son is given; and the government will be upon His shoulder. His name will be called the Angel of Great Counsel, for I shall bring peace upon the rulers, peace and health by Him. ⁶Great shall be His government, and of His peace there is no end. His peace shall be upon the throne of David and over His kingdom, to order and establish it with righteousness and judgment, from that time forward and unto ages of ages. The zeal of the Lord of hosts shall perform this.“
Excerpt from: "The Orthodox Study Bible: Ancient Christianity Speaks to Today's World" by Thomas Nelson.
This is one of those rare cases, like Psalms 1:12 , where the Masoretic preserves a Christological reading absent from the Septuagint. Usually its the other way around.
Personally I would prefer a blending of the different extant versions of the Old Testament to maximize the Christological content and ensure the text quoted in the New Testament, for example, 1 Enoch in Jude, is present in the Old, so that the interests of the Church would be best served. It is not well served when I as a minister have to remain familiar with five different major recensions of the Old Testament in five languages, and four recensions of the New Testament in two languages.