The Bell Jar is a very unusual book in that it had three different first editions by three publishers within a decade!Of course the first was the 1963 Heinemann edition by "Victoria Lucas".
The second was in 1966 by Faber under Plath's name.
And the third was the 1971 US edition by Harper & Row.
Only the US edition(s) have the Afterword by Lois Ames. Similarly, the US edition of Ariel featured a foreword by Robert Lowell. The UK edition, which preceeded the US edition by several months, has a slightly different order, and no foreword.
Careful readers of Plath's poetry, notably poems in The Colossus (1960 Heinemann/1962 Knopf), would have noticed the similarities between two of the poems in that collection and some scenes in The Bell Jar. Those two poems were "Suicide Off Egg Rock" and "Point Shirley".