Ballantine Dupe Numbers
Ballantine 269 & 269K
Ballantine 476K & S476K
These pairs of books bear the same book numbers. I'm not sure what prompted the screw-up, but it may have had something to do with the implementation of the "K" suffix (mentioned in a previous "Oddity"), which first took effect with #267.
To my knowledge, these are the only duplicated numbers in the Ballantine run.
The Bilko Joke Book was supposed to be numbered 298K, according to Holroyd (but that may have been an error on his part). I have never seen a copy or a scan of a book bearing that number, and I am assuming that none exists.
As for the two books numbered 269: there are no known Ballantine numbers 270 or 271. It is probable that The Key to Economic Progress was originally meant to bear one of those numbers. It is not listed in any vintage paperback price guide. (I own that book, and there is absolutely no doubt that is the correct number.)
Both scans of 289K are courtesy of Daniel Snethen.
The scan of 269K is courtesy of Brian Maginnity.
The scan of S476K is from Dave Biemann, who pointed out yet another number duplication.