English: 1955, per the copyright notice. Note that the film itself was released in the U.S. in January 1956.

English: Scan via Heritage Auctions. Cropped from original image and retouched by uploader; see unretouched version in upload history.

English: "Copyright 1955 Paramount Pictures Corporation"

English: The poster included a valid copyright notice, seen at bottom left. However, the copyright for the artwork was not renewed, as was required by American copyright law to extend/maintain protection for works published 1963 or earlier. See the Copyright Catalog (1978 to present). In order to maintain copyright protection, the poster would have had to be renewed 28 years after publication. Because it was not renewed, copyright lapsed at that time. Note that the poster art is a distinct work from the film it represents and had to be renewed separately.