I guess we can also rule out the possibility of it being a '81 revision just by going on the Dolby System logo, I'm not sure exactly when it became known as Dolby Stereo, but at the time of Ridley Scott's Alien in '79 it was already in place, so if they had made new credits for the '81 re-release it would most likely had Dolby Stereo on it.

These two Dolby System logos also have a variation in the font used for "Making Films Sound Better", "Noise Reduction - High Fidelity", but I think these two logo variations were in use at the same time. The one on the 2006 DVD is seen on most posters for the film at the time, even teasers.