Whew, had me worried, there for a second. I agree, just to get it working as originally intended. I don't have a Davis or Minnesota, but do have several long shuttle machines. Some have the spoon shaped tension release and some that have a pin in the center of the tension assembly that pushes the tension discs apart a bit when the presser foot is raised.
Have fun with your "new" machines.
Janey - Neat people never make the exciting discoveries I do.