Wikipedia: https://en.wikipedia.org/wiki/Regnery_Publishing
Archives: Henry Regnery papers, 1909-1996, Stanford University, Hoover Institution Library and Archives, https://oac.cdlib.org/findaid/ark:/13030/kt9v19q4s4/.
Henry Regnery started the company bearing his name in 1947, and throughout most of the time Rengery himself controlled the firm, its focus remained on conservative ideas and figures. Funny enough, its perennial bestseller in the early 1960s was its release of Marx's Communist Manifesto (#6008, 1955).
The Henry Regnery Company changed its name to Contemporary Books, Inc., effective May 1, 1977. By that point, Henry Regnery himself had been absent from the company for some time, according to then-president Harvey Plotnick, and the name change reflected a departure in direction from the Regnery-less company (All this according to Regnery Changes Name, Announces New Programs, PW, Apr. 18, 1977, at 28).
By the end of 1977, Henry Regnery had purchased the paperback backlist from his former firm, now Contemporary Books, Inc., and had formed Gateway Editions, Ltd., based in South Bend, Indiana. This newly-formed company republished many of the Gateway titles from the Regnery Company backlist (see Gateway Editions, Ltd., ad in PW, Aug. 29, 1977, at [101] for a full list of titles with ISBNs.) I'm not concerning myself with this Gateway 2.0, as they seem to lack any semblance of order as a series. By early 1979, Regnery had changed the Gateway Editions, Ltd., name to Regnery/Gateway, Inc. The firm continued to offer Gateway Editions well into the 1990s.
Paperbacks: A Report on 1960: Mass Market and Higher-Priced Fields, PW, Jan. 16, 1961, at 72, 78 ("Regnery, hardly a left-wing firm, found itself in the interesting position of having its leading paperback bestseller in the Gateway Editions line in 1960 Karl Marx's 'Communist Manifesto,' which sold approximately 19,000 copies.")
1962 Paperback Best Sellers: In the Bookstores, PW, Jan. 21, 1963, at 50, 57 ("Regnery's top seller during 1962 in the Gateway paperback series was the perennial 'winner,' 'The Communist Manifesto,' of which approximately 53,000 copies were sold.")
Regnery Changes Name, Announces New Programs, PW, Apr. 18, 1977, at 28.
Gateway Editions, Ltd., ad in PW, Aug. 29, 1977, at [101].
News of New Publishers: Gateway Editions, PW, Nov. 21, 1977, at 22.
Name Changes: Gateway Editions, Ltd., PW, Mar. 5, 1979, at 29.
Henry Regnery, MEMOIRS OF A DISSIDENT PUBLISHER (Harcourt Brace Jovanovich 1979), https://archive.org/details/memoirsofdisside00regn/page/n5/mode/2up.