Ballantine Books, 1970. Second printing, paperback. First published in 1965, this is a collection of three tales by a master of "hard" science fiction: Assumption Unjustified (1946); Technical Error (1944); Impediment (1942).
Used availability for Hal Clement's Natives of Space
April 1965 : USA Mass Market Paperback
1970 : USA Mass Market Paperback