Midland Books

From PW, it appears that IU Press published Rolfe Humphries's translation of the Metamorphoses simultaneously in cloth and paper in April 1955 and then issued it later, in 1957, as MB-1 in its new paperback line, Midland Books. 

INDIANA UNIVERSITY PRESS . . ., PW, Jan. 29, 1955, at 705 ("Indiana University Press is venturing into the field of simultaneous cloth and paperbound editions with the publication in April of Rolfe Humphries' new translation of 'The Metamorphses of Ovid.'" 

Spring Books from Indiana, PW, Jan. 28, 1957, at 63 ("Announcing . . . Midland Books. A discriminating collection of paperbacks—including previously inaccessible classics, as well as works from Indiana's own backlist.")