My previous post described my confusion as to how the lockup bar for my Challenge proof press works.
As I mentioned in a comment to that post, I found my answer online in US patent number 3288062 which shows how the parts are assembled and how they work.
After straightening out the extra bend in the locking lever, I reassembled the lockup bar as shown in the diagrams attached to the patent, and adjusted its length so that when the lever was in the released position, the bar could just be swung in and out of position without jamming against the rails.
With this adjustment done, it worked just as described in the patent. Once the length of the bar has expanded to be tight between the rails, further movement of the locking lever causes the pressure bar to bulge out, effectively tightening up the lockup.
There is one other lockup-related thing to do with this press, which is to make sure the posts placed in the bed when using the dead bar for lockup can be easily removed without having to yank them out with Vice-grips!