Ah, the video really helped. On the 68, the catch bit is holding the spring higher than the blade tang in the closed position. This is either from the factory that way, or the tang on that pen blade has worn down enough to produce a gap. The tang on the pen blade is not uniform between the open and closed position, that is why it's tight when open, sloppy when closed.
PS. check to see that the catchbit is not askew in the frame, that could be making the spring ride higher than usual. Is the spring flush with the liners in the closed position?