Swallow Enters Quality Paperback Field, PW, Oct. 19, 1959, at 23 (announcing first 5 titles for publication September 30, 1959, with 2 more titles in December 1959 and another in January 1959).
Series continued past 11 titles, but it's apparent that the loose serial numbering (only indicated on the back covers of the early titles) was abandoned at some point in 1960-1961.