[TUHS] Copyright notices in Edition 5, removed in Edition 6

Apologies if this has been answered before, but I noticed that there are AT&T copyright notices in the kernel sources for Unix Edition 5, but they were removed in Edition 6. You can still see the comment blocks for the notices in Edition 6, but the notices themselves have been removed. Does anyone have the history on this? I noticed that USL registered Editions 5, 6, 7 and 32V in 1992. I would assume that Editions 4 and earlier are free and clear because, prior to 1978, registration was a requirement for protection. Further, since USL waited longer than 5 years to register the copyrights for 5, 6, 7 and 32V, these may be free and clear as well. As I understand it, Editions 7 and 32V could have had copyright protection without registration since they were released after 1978. However, because they lacked copyright notices when released, they may very well be considered public domain. It was not until 1989 that the requirement for including copyright notices was dropped.