As I recall, it came from a notation by a coypist made in the margins and later entered into the text. Hopefully, someone else will have more information for you
It was added by the Catholic Church in the early centuries. I’m not sure if it was by the action of a Church Council or simply a Liturgical act. I do not believe you will find it in any version of the Bible except the KJ.
I do not think that it is correct to call this a “second part of the Lord’s prayer.” Rather it is a doxology which has been *liturgically *associated with the Lord’s prayer from the early centuries. This is the reason that the two prayers are separated in the Mass, as a means of making clear that they are distinct, while honoring the ancient tradition of adding the doxology in public prayer.
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