From a CARM discussion:

CARM - Biblical Languages
Initial Considerations of the Use of KURIOS in Acts
https://forums.carm.org/vb5/forum/theology/general-christian-topics/biblical-languages/5070962-initial-considerations-of-the-use-of-kurios-in-acts?p=5083391#post5083391

There is a lot of material on this from the Messianic_Apologetic discussion days.
I'll try to expand the discussion on this thread.

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If the KJV has the convention of treating LORD differently from Lord, and the first is supposed to reflect the OT use of the Tetragram, and the second use is supposed to reflect "Adonai," then their own translation of Acts 2:34 disregards their own convention:... I am beginning to believe the KJV completely arbitrary and GUESSING about their own translations. Steven AverySt
And I suggest that you use the Pure Cambridge Edition of the AV, which functions as a type of Received Text of AV editions. It is online in a PDF, however I have SwordSearcher software, by Brandon Staggs, which uses the PCE.

Acts 2:34-35 (AV-PCE)
For David is not ascended into the heavens:
but he saith himself,
The LORD said unto my Lord,
Sit thou on my right hand,
Until I make thy foes thy footstool.

Matthew 22:44
The LORD said unto my Lord,
Sit thou on my right hand,
till I make thine enemies thy footstool?

Mark 12:36
For David himself said by the Holy Ghost,
The LORD said to my Lord, Sit thou on my right hand,
till I make thine enemies thy footstool.

Luke 20:42-43
And David himself saith in the book of Psalms,
The LORD said unto my Lord,
Sit thou on my right hand,
Till I make thine enemies thy footstool.


This is the same consistency as in Psalm 110.

Psalm 110:1
A Psalm of David.
The LORD said unto my Lord,
Sit thou at my right hand,
until I make thine enemies thy footstool.


You will find that the corruption versions frequently have this wrong, OT and NT.

And the Jewish "anti-missionaries" have understandably used this as a point of attack on the Christian versions, especially on Psalm 110. I have seen them attack the AV-1611 on Psalm 110, hoewver that distinction was clearly shown even in the original 1611 edition in the Psalm verse.

AV-1611 - Psalm 110
http://sceti.library.upenn.edu/sceti/printedbooksNew/index.cfm?TextID=kjbible&PagePosition=704

Steven