Library of Congress reports "[1964, (c)1965]": that is, 1964 publication inferred or researched, not stated on the title page; copyright 1965.
Not found 2018-08-02 in automated search ('nathaniel benchley' 'the visitors') of newspapers prior to several reviews and notices published February and March 1965.
1st ed.
Library records (LCCN and OCLC, below) reports "McGraw-Hill [1964, (c)1965]" --that is, publication before copyright; inferred or researched, not stated on the title page.
As of 2018-08-02:
Amazon.com as "McGraw-Hill; 1st edition (1965)" provides image of plain grey cloth front cover with silver or white lettering on the spine, illegible.
Amazon UK provides a front jacket image as" Hutchinson; First Edition edition (1965)".
--both as ASIN B0000CMS8Y
Price and cover artist (as "Chas. Addams") on front flap of archive.org copy; changed date to 1965 as per copyright page.
