The part I need is a part for the thumb rotor. It is a plastic connector that connects the thumb lever to the rotor stop arm. The rotor string goes through it plus it has a hole in one end that a screws goes through and connects to the thumb lever. The other end has a threaded hole, which is broken on this one, that screws down onto the string. This horn was made around 1978 -1979. I will probably have to fabricate the part but it would be handy to just replace it if I can find this "needle in a hay stack".
Click on the link below for a photo of a horn with the needed part shown. I cropped it to get a better view.