John, you were right. Fore some reason it was trying to open as text. Winner. Thanks so much of your help. You have made someone very happy today. Thanks again for taking time to reply!
As a followup, another sign that the file association has been damaged is if you select the file in Finder and the preview takes a long time and then shows gibberish. Same problem (the system is attempting to treat the SketchUp file as text) just seen in a different place.