If you can find a 1/4 in. 20 screw with a knob on it, just chuck it up in a drill with the width of a file from the knob. Turn on the drill and with the narrow flat side file the threads off down to the main body, un chuck and you have a screw and a knob that will slide along in the T pod head. Most Tri pod heads have one end of the slot threaded so the screw can be put in. If yours dosn't, find some one with a 1/4 inch tap and and thread the end of the slot. Usually when one gets lost it is because it unscrewed itself from the head.
Charlie..............................