The Letters of J.R.R. Tolkien.
Edited by Humphrey Carpenter
with Christopher Tolkien.
1st Edition 1981.
George Allen & Unwin.
ISBN 0048260053.
Hardback in dustwrapper.
Hammond pp.350-351.

Three states of this edition exist:

1) Pages 406-407 and 427-428 imposed in reverse order. (These were distributed to reviewers before the error was noted)

2) Pages 405-408 and 427-430 replaced with correct pages attached to stubs of original pages.

3) Pages 405-408 replaced with correct pages attached to stubs of original pages. Pages 409-430 completely replaced with new pages. Two variants of this have been seen - one with the replacement pages made from the same paper, and the other on smoother, lighter weight paper.