Around 1961, for a guess, Pocket Books raised prices on backstock from 25 cents to 35 cents. They put
updated covers on the same printings of text blocks, so there are titles where the same printing
number, as recorded on the verso of the title page, appears under to different prices and book numbers.
The Case of the Long-Legged Models is an example. Both copies below are first printings from June 1960.
The only difference found in
6009, aside from the price and book number on the front cover, and the book number on the spine, is
a Detective Book Club advertisement bound in between pages 88 and 89 - for the six latest mysteries by
Erle Stanley Gardner plus a detective book club triple-decker. The six mysteries run from The Case of the Calendar
Girl from 1958 though the Case of the Duplicate Daughter published in June of 1960. That suggests the switch
was made in late 1960 or early 1961.