Earliest legible date stamp is October 1955. There is a date stamp on the original image that appears to say "September 4, 1967". Dunn died on September 1, 1967, and this image was used in some newspaper obituaries. Photo appears to have been taken in mid-1940s, when Dunn was cast in 20th Century Fox's 1945 film A Tree Grows in Brooklyn.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.