Hi Dave,
Don't want sound contrary, but I am sure Steve flew his Cobra at the '66 Worlds. That is the plane in the rafters in the Museum Hobby Shop. One of the very nice younger guys put on white gloves and got a ladder to get it down so I could take pictures! It has the FAI sticker on it.
Steve did fly the Argus at The Worlds but not the published one, the WC Argus was voted the most beautiful model in the World and was slightly bigger, not by much, from the published model.
I really researched the first three WC's '60, '62, and '64 And have built three of the models flown in them. I was actually research Mr. Grondal's Nobler (He called it the AMA Special) while he was still alive and had several nice conversations with him! He won the first two official WC's!!
Bill