PD-PRE1978. The photo has no copyright markings on it as can be seen in the links above. Note that I have enlarged and included an image of the mark on the photo's lower left corner. It is a number 3 and not a copyright mark.
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.
This image has been assessed under the valued image criteria and is considered the most valued image on Commons within the scope: Vivian Vance. You can see its nomination here.
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