Sorry that I still keep talking about this sequence, but it fascinates me. ;) Here's the two versions of this shot for an easy comparison:

X-Wing takeoff: http://www.sendspace.com/file/p1g9to

X-Wing takeoff 2: http://www.sendspace.com/file/c0u4f3

I'm leaning towards the cloudless composite being another '81 re-release modification, as so far every '77 version contains the cloudy composite, if this is the case it also reinforces the theory that the two different credit rolls existed already in '77. So, why did they bother re-composite this shot? Well, I think it might be down to the first few frames missing the clouds, in high resolution on a big screen, the optical clouds suddenly appearing on the fourth frame might be quite noticable and distracting, especially when you are the filmmaker. When I now have pointed it out I may have ruined it for some of you. ;) But take a look at the clips and compare for yourself.

Now, why did the '77 shot reappear on the NTSC '80's home-video master? I guess it just slipped by being such a minor difference, the source of that video master is still a little mystery though as it doesn't contain the print damage on the blockade runner like the release prints did. Would be interesting to know which composite is included on the various PAL video releases.