Leverage.

On an O frame type press,where is the best place to pivot the swing links.I used to think the pivot should be on the centreline of the ram,but now I think the pivot should be on the operator/ lever side,so that side load on the ram is reduced near overcentre position.This position also allows a single long pin thru the frame to be used.

Just look at the compound linkages on RCBS or Redding presses. That should answer your question.

Randy

On the center line at the very top of the frame.
Of course the links would be in your way.
By pinning the links to the top of the frame like the Redding Ultra Mag only the top of the frame has any real stress on it.
Some times design theory and ergonomics have to reach a compromise though.