Some Spring Highspots January-May: Religious Books, PW, Feb. 14, 1966, at 84:
This is one of six books in a new paperback series, Logos, which Regnery will launch on February 28, each priced at $1.25. The series is, the publisher explains, an outgrowth of Vatican II. It consists of both originals and reprints.
Although the above announcement for Chenu's The Theology of Work (#708) indicates the book would arrive February 28, 1966, it didn't appear in PW's Weekly Record until the Aug. 22, 1966, issue. Multiple Logos titles by that point had also appeared earlier in the Nov. 15, 1965, issue's Weekly Record. It does appear that the first titles are from 1965 rather than 1966, as indicated in PW.
The format for the series numbers is also peculiar. The earliest Logos titles recorded in PW have series numbers from L704-L707. The next titles to appear, in 1966, have a new format: 51L-708 to 51L-713 and then 6L-714 to 6L-719. Title #720 appears not to have been published, but titles from 1967 and 1968 have a new format: 61L-721 to 61L-725 and then 71L-726 to 71L-731. Titles from #701-703 appear to have a similar format, 41L-701 to 41L-703, although I've also seen the "41L-" prefix used on #704. The prefixes are clearly sequential, but I can't figure out their significance.