Alan Sauer's documentation about the Nine Companies of Ringwraiths [see previous theory] clears up a number of anomalies sprinkled through CJRT's reprints of JRRT's works.
For starters, it explains the altered passages in Return of the Shadow (pp. 257-259) where Gandalf refers to the "twenty-seven rings of the Nazgul" (emphasis mine), a number evidently amended to eighteen after the first draft of Frodo's flight to the ford (Treason of Isengard pp. 61-2). For those unfamiliar with it, JRRT tried eight times to get Frodo across the ford and failed bitterly every time. "I have got the hero into such a fix that not even an author will be able to extricate him without labour and difficulty," he wrote to Stanley Unwin at the time.
A later solution to the still-excessive number of Ringwraiths is reflected in a pair of isolated passages in Unfinished Tales (pp. 347-8 hardback). In these, JRRT says that "the Enemy originally had sent half the Nazgul to the East, to an Easterling city called 'Bakensh', evidently unsure after his interview with Gollum whether he was seeking a person named Baggins in a place called Shire or a person named Shire in a place called Baggins". This accounts for the very strange manuscript in the early paperback editions of The Tolkien Reader (up to Ballantine's eighth printing, March 1969), called "The Ballad of Ahmed Shaire" which deals with the pursuit and eventual downfall of an Arabic swordsman who was tracked and killed by "nine horsemen of night"(!).
"'Go back!' Shaire's voice challenged bold.(From "The Ballad of Ahmed Shaire", 251st stanza.)
'I have it not, your Ring of gold!
Go back to dark lands of the West!
The failure of your cursed quest
And anger of your precious Lord
Will not compare to what my sword
And will of iron may do to thee.
I urge you, evil Riders, flee!'
A chilling voice replied in scorn.
'Thou dare defy us, thou baseborn?
Your squeaky voice doth make us laugh.
As we should fear your puny wrath
Or hesitate before your blade!
If thou of sterner stuff were made
Then thou our wills you would command.
But even now I see your hand
Would quake before it held the Ring.
Tell us where you have hid this thing,
And maybe we shall let you live.
Perhaps the Eye will yet forgive!
But hesitate, defy again,
And here with other mortal Men
Shaire will never tarry more!'
And down on him the Rider bore..."