Martin,
Not the best design as it pushes so it's only relying on the two dovetail faces.
If you want to keep it simple, scrap the two push screws in the post and fit two screws in the holder one above the other.
Drill and tap them in the face just above the OL in holder, this way it will grip on 4 faces and pull back in to the dovetail.
A better design would be to reverse the dovetails, dril the same toe screw holes as above but now drill them into the post and fits a gib strip for the screws to push on.
Advantage of this is only two hles to drill and tap instaed of in every holder and the holders wil be easier to make given that they are open dovetails which means you can use angles side and face milling cutters instaed of fraginle dovetail cutters.

Edited By John Stevenson on 15/09/2012 23:43:18