I'd guess that the reason it's jammed is due to the corrosion of the h-bar expanding inside the rigid grip. if that's a steel bar (as it appears) then maybe try some mild acid solution if you can get it to wick in around the gap and loosen the rust. I'd try white vinegar before anything stronger...what that will do to bone, I'm not sure. Might bleach it a little...try a test on something like a hambone.