Just scanned this picture from a magazine of one of the pre production prototype cb750s. You can see all sorts of differences (carbs, steering damper, sidecovers, clutch cover, master cylinder, etc). I'm guessing this is the same bike that 736cc was posting about finding a couple years ago. You can see it better by clicking the direct link.
enjoy...
http://i158.photobucket.com/albums/t110/hoodellyhoo/Scans/cb750prototype.jpg
