This is more than a rumor. George Lucas made the following comment in the book "Once upon a Galaxy: a Journal of the Making of Star Wars, The Empire Strikes Back, by Alan Arnold(1980):
"Walter Murch and I were mixing American Graffiti, and Walter wanted to make some changes in one of the sound tracks. He needed the dialogue tracks for reel two so he said, 'George get me R2-D2', meaning reel two, dialogue two ... and I said 'that's a great name, Walter' and wrote it down in my notebook" (pg. 224)
"Walter Murch and I were mixing American Graffiti, and Walter wanted to make some changes in one of the sound tracks. He needed the dialogue tracks for reel two so he said, 'George get me R2-D2', meaning reel two, dialogue two ... and I said 'that's a great name, Walter' and wrote it down in my notebook" (pg. 224)