I don't think PPS was used for the first titles. They seem to have just been numbered without a prefix. See https://www.ebay.com/itm/384657789191?hash=item598f641107:g:vBQAAOSw6pxh1On8.
New Line of Praeger Paperbacks, PW, June 15, 1959, at 110 (announcing the Sept. 1, 1959, launch of Praeger Paperbacks, including the first 9 titles in the series).
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.