Chewtobacca said:

So does that mean that the JSC comes from an extremely early source that was made before the negative was damaged? I'm still as confused as ever about this.

You're not alone. :) We don't have a definitive answer, we can only speculate, but yes that appears to be the case.

It's also worth pointing out that the opening shot of the Tantive corridor is an optical effect shot, so the workflow went: negative --> IP --> IN (cut into the negative of the rest of the movie) and then --> IP --> IN --> release print.