To clarify, there are two ways that complete synch could be restored for RotJ: either the two frames at the end of the shot could be deleted from the video, or the audio could be extended to the proper length. The latter choice would be more 'authentic', and it's probably easier for me to do it so that the whole projects don't have to be re-rendered. I'd just need a fresh rip of the '93 laserdisc PCM for that section to fill it out.
Something crossed my mind recently about the in-theatre 70mm recording: if the surround channel wasn't running, then it stands to reason that they didn't have dedicated subwoofers installed, either. Since SW was the first film ever mixed with separate LFE, it's safe to assume that most theatres when it first came out had not yet upgraded their equipment to play it back at its absolute best. The boom tracks were intended to be played through channels 2 and 4, the ones in between the LCR fronts, and these may still have been ordinary speakers in some locations. If that is the case, then the bass may not always have been as strong, since LFE is deliberately recorded at -10 db from the intended playback level to improve headroom, and compensated for at the amplification stage. This 10 db boost at playback may not have been implemented if a particular theatre was not aware of the requirement, or if their equipment couldn't handle that high a level; in such a case the bass would probably come across more like it does in the '93 mix—definitely there, but not palpable in the same way.
I was thinking about this lately, because if the 70mm bass was really as loud as I think it was, then the recording may have ended up being much more clipped and distorted than it actually is in those parts. The above conjecture, as well as acoustic concerns, could easily explain this discrepancy, and of course we should not forget that the recording is very old and probably didn't have good low frequency retention anyway. Still, while I'm pretty sure I've estimated it correctly, sometimes I wonder if I might be mistaken. Hmm . . .
Anyway, now to get back to working on the mix instead of just talking about it. ;)