Hmm, can't remember where I've read about when exactly the mono prints debuted... have to find that.
From the 2004 DVD audio commentary Ben Burtt says:
...the first mix done was a two-channel stereo Dolby mix wich hadn't been done before, it was a new process. That mix was done first because we were actually making prints and testing them out, and getting theaters to tune up their equipments required that mix was done first. After that was done, then a six-track stereo mix was done for the 70mm release of the film and of course going back and mixing the film a second time, one make changes, you take things a little further you've learned from the first mix of the movie and you now go and make all your changes in the next mix wich was the six-track...
I think the above quote along with the minor additions and differences in the six-track mix which also appears in the mono mix confirms that the 35mm Dolby Stereo mix was done first, but the 70mm six-track which was derived from the same source was first out the door... It makes no sense to remove certain sound effects in the 35mm Dolby Stereo mix then add them again for the mono if it was the second mix.
Anyway, I find this all very fascinating. The revised credits are a little bit of Star Wars history that have gone largely unnoticed. :)