I just got to thinking...did you try to turn the adj. screw BOTH ways? I wonder if someone turned it to it's end and the adj. screw is now tightly turned into the clevis. When you had the double nut on it to get more leverage, did you try to turn it both ways? I think the adjusting screw has right hand threads on the inside portion and if it got turned all the way to the right hard, it may have just locked it into the clevis. I'd try to turn it counter-clockwise first to see if it comes loose. I just can't see how the two can be locked together unless someone tightened them together. Try clockwise too if the other way doesn't work.