The original chapter in 1830 consisted of the material in our current Chapters 23 through 27. Our current edition correctly notes that the account of Alma only covers the material in our current Chapters 23 and 24. Orson Pratt recognized that Mormon returned to the large plate material in our Chapter 25. Mormon does not indicate when he returns to that source, though he has previously created a chapter break when he does. In this case, we can discern that return through the nature of the text rather than the explicit creation of a chapter.
The historical material that Mormon uses for our current Chapters 23 and 24 came from the separate record of Alma and consists of his record of his people. Mormon had previously referenced this information, so we know that he had read it and planned to include it at this location.