I was going by what this caption said on Corbis. But then, they do say early '70s. And since you were there, you'd know! Were you at all six nights they performed in LA?
http://pro.corbis.com/popup/Enlargement.as...8e-241615bfa6b6}
If it were '73, he wouldn't have the scar. He must have been feeling really hot that day to bring it out that top!
edit: Search for Robert Plant pictures on http://pro.corbis.com/search/searchFrame.aspx . They have a few great Jazz Fest pics!