Thank you very much Michael, for your response.
It is very understandable, and useable. :-)

(The mirror script is the official moi script in its command subdirectory.)
So the x-axis and y-axis direction vectors used to create the new frame would be (or could be) in world coordinate frame.
A new y-axis, parallel to the world coordinate y-axis would have the same vector direction, (0,1,0).
The new x-axis vector direction can be calculated from the two points, mathematically, or with vectorMath...