Milestones of Thought
Milestones of Thought launched March 1956 as an unnumbered series for the college market, at 4 3/4" x 7 3/4". The first four titles were Calvin's On God and Man, Rousseau's The Creed of a Priest of Savoy, Dostoyevsky's The Grand Inquisitor, and Darwin's The Origin of Species, the first three of which were priced at $.65 and the Darwin at $.95. Clothbound editions were simultaneously published at $1.75 and $2.25, respectively.
At some point in the early 1960s, additional printings included series numbers, which don't appear to have been related sequentially to the original order of publication. The first title whose series number was noted in PW is M114, Discourse on Free Will, reviewed in the July 10, 1961, issue of PW. The series continued with numbers until at least 1967, when M130, Deism and Natural Religion: A Source Book , was published. From that point it appears the titles did not have series numbers.
One title was published during the original unnumbered run that I haven't found with a later series number: Pliny's Natural History (early 1957). It's possible it was republished as M117, for which I've found no information, although this title was certainly later republished in the early 1960s under the number 1101 as an Ungar Paperback.
Over the years, more titles were published in the Milestones of Thought series with no associated series numbers. The first batch was from 1970-1971: Selected Writings of Jonathan Edwards (late 1970), Matthew Arnold's Literature and Dogma (late 1970), and Cotton Mather's Magnalia Christi Americana (early 1971). Another round of titles came out in 1979: Robert Burton's The Anatomy of Melancholy (1979), Samuel Johnson on Literature (1979), The Speeches of Pericles, from Thucydides (1979), Hegel on the Arts (1979), and John of Salisbury's Policraticus: The Statesman's Book (1979). Finally, two late entries from 1987-1988: Emerson on Transcendentalism (1987) and Paine and Jefferson on Liberty (1988).
Frederick Ungar's Paperback Series for College Market, PW, Feb. 18, 1956, at 1041-42 (announcing launch of Ungar's Milestones of Thought series, the first 4 titles of which, ready in March 1956, were Calvin's On God and Man, Rousseau's The Creed of a Priest of Savoy, Dostoyevsky's The Grand Inquisitor, and Darwin's The Origin of Species.)