ace books mmpb second printing
half a double with a single front cover;
i.e. not a classic "ace double" with two front covers and texts printed upside-down with respect to one another
also second ace mmpb printing of this pairing
cover art by attila hejja (miscredited as attilla heja)
cover price $2.75
320pp. including 2 x titles, indicia etc, end pp. advertising
derek calver #1, doubled with #2: "the ship from outside" (1963), q.v.