Supplements would be issued when new features were added for later models , like Air Con, or for a different market or further service instructions incorporated. Later the whole manual would be revised and reprinted as a whole. It especially happens in the early life of a model . I have early SD1 manuals and there are two supplements to the main manual.
Another thought has come to mind, one must remember , the 1970's were the early days of computers and frequent & last minute changes to printed books was not easy and very expensive as the printing plates were fixed . I am a member of the Rover sports register and we reprint the Pre-war Rover manuals, and can now get them economically run off in batches of only 30 or so, whereas only 10 or 15 years ago it was in the hundreds