Praeger University Series
Praeger Launching New Paperback Text Series, PW, July 18, 1960, at 49 (announcing the new Praeger University Series and noting that the first two titles had already been published in the Praeger Paperbacks at trade discount but will instead only be available at short discount).
Discounts and Returns, PW, Jan. 8, 1962, at 74 (announcing that Praeger "is dividing its Praeger Paperbacks into two series, which will carry different discounts," Praeger Paperbacks, using PPS code letters, and Praeger University Series, using U code letter).
I thought this already happened in 1960? I did find one title, The American People and Foreign Policy, whose Weekly Record entry, PW, Jan. 16, 1961, at 160, lists it as both PPS-38 and U-504. Based on the above article, my guess is that Praeger for some time each title in two versions: one as a PPS title at retail and one as a U title in academic bookstore. They must have in 1962 divided the lines so that titles didn't appear in both lines going forward.