Letter link: Letter from J.R.R. Tolkien to Forrest J. Ackerman • c. April-May 1958 (#1100)

Why this one is a separate entry and not Carpenter #210: Letter from J.R.R. Tolkien to Forrest J. Ackerman • 9 June 1958 (#584) ? Letter 210 claims to be "undated", but Chronology and Marquette say "9 June", is Letter 210 a draft then?