The casting for the locking mechanism is not "exact" but my Columbian which is similar to yours, maybe exact, has never been reliable with locking, unlocking, or sliding. It ised to work to the point of being functional, but when I moved the bench out to the garage I removed the face off the vise and when I reinstalled it it became nearly unusable. Never liked it and I will probably replace it. However, I will watch this thread with interest.
One thing I notice right away from the pictures is that the locking collar with the hinge is I believe, upside down. It needs to be rotated almost 90 degrees around the threaded shaft and the Angled portion of the small ear has to be engaging from the bottom on those ribs. You can see the wear marks there. If that doesn't work, then it needs to be flipped around. I'm pretty sure the smaller angled ear has to be engaging the rib with the angle, not the straight part of the ear. The other larger ear I think just nests up inside that cut out.
Have you disassembled the vise or is it as you acquired it? Previous tinkering and incorrect assembly may be to blame here.
However, you have good reason to not take my advice because I just said, mine doesn't work worth a lick anymore. It did at one time though. I bought mine in pieces though and had to trial and error it back together through many non identical internet pictures. I stumbled upon one picture of the underside of one finally though and all became clear and that was the time it worked. I'm pretty sure that locking collar hanging down like that is flat out wrong though. Even unlocked, it has to remain up in that little nested area. I should also note I had to fabricate a rod end plate. The plate with three holes that is held on by a roll pin going through the end of the threaded rod. I may have gotten the geometry off ever so slightly and is just now coming to fruition.
Also the locking collar has an arrow on it and the word "front". Which way is front exactly, I don't know. I think I faced mine towards the face of the vise.
Here is my vise. And here is the underside of it.
