The Strange Case of the Ballantine "K"
Ballantine used prefixes as part of their numbering system to designate various prices, such as an "F" to show a 50¢ book instead of their regular 35¢ cover price. Many paperback publishers used systems like this.
But beginning with book #267, a letter "K" suffix began appearing. It continued, almost uniformly, until book #527, when it disappeared just as mysteriously as it appeared. I have no idea why it started, why it was there, or why it stopped.
There is no "K" on book #445 (and possibly one or two others).