This series has a highly unusual numbering system. The first five titles, published April-May 1960, appear to have the numbers 501, 502, 1102, 1103, and 2101. I think the reason 1101 was not advertised as one of the first titles is that the title, Pliny's Natural History, was earlier published as a title in Ungar's Milestones of Thought series. The edition published as an Atlantic / Ungar Paperback is probably just a second printing/edition of this title.
As best I can tell, these are the bases for Ungar's separate numbering schemes:
500s -- works on religion, spirituality, and philosophy [1960-1967]
1100s -- books on history and world affairs [1960-1963]
2100s -- literature, mostly German [1960-1967]
3100s -- science guidebooks [1961]
5100s -- titles in self-help and entertainment [1960-1962?]
It's difficult to tell if there are other sub-series or indeed to find much information about the Atlantic / Ungar Paperbacks, as Ungar evidently didn't advertise much in PW, and their works are rarely reviewed in PW, especially not past 1961 or so. The lack of a series number prefix makes searching for titles arduous. Ungar also did not provide much publication information beyond copyright dates on the verso pages, which means sources like WorldCat do not have the actual dates of publication.
Some of these titles were previously published in the unnumbered Ungar series College Translations, which began publication in 1955 (see PW, Feb. 18, 1956, at 1041-42.)
Frederick Ungar Lists New Atlantic Paperbacks, PW, Mar. 21, 1960, at 31.
Ungar Enjoined from Using "Atlantic" Imprint, PW, Aug. 7, 1961, at 34 (proclaiming results of lawsuit between Atlantic Monthly Press and Ungar, by which Ungar was enjoined from using the name Atlantic Paperbacks).