@My Friend Jack - I expect the only contents ‘incorporated’ from Rock File 1 would be the British chart data (Top 20 hits 1955-69). The listing is brought it up to date (end of ‘74), adding Billboard Top 20 hits as well. I don’t have Rock File 1, but the accompanying articles in Rock File 4 were clearly written circa 1975 (including album reviews of Elton John, 10cc, Barry White, The Eagles), and would not have been in the original Rock File. I picked mine up soon after it was published too.
I remember getting hold of this as soon as it came out - it was the first time that an data on US chart history had been easily available. I don't recall the "incorporating Rock File 1" bit being on mine, though, and I do find that a bit of an odd subtitle. The original Rock File contained a number of articles and essays as well as the UK Top 20 Log 1955 to 1969, but what was in RF4 went quite a way beyond that. There were several articles in RF4, as I recall, but I don't believe they were the same ones that were in RF. Presumably they wanted to stop people from trying to track down copies of the original book.
If only I knew in which box in the loft I'd stashed my copy many years ago...