There is a long list of the differences between early Sandcast and later K0 documented here on this site. Lots of very good info.
http://cb750sandcastonly.com/ As you begin to tear down your bike for cleaning and tuning etc. I cannot stress enough to be very careful removing your carb rack. Use a heat gun on the rubbers and take it very easy. The carb mount plate can be brittle and was a one year only deal and snapping it in the middle would make for a bad day. Whats left of the hard to find airbox will be brittle also by now.
Check that upper triple tree and make sure both sides have a 'D' washer in the gap where the bolt goes through. Looks like you have a replacement bolt because of the fairing bracket and guys lose them or I have seen them mounted on the outside before where they do no good. Breaking that makes for a bad day also
Be proud of your find and keep us up to speed, were here to help, not taunt.