Thanks for the photos. Some later bottles of Dewar's include a date on the reverse side of the label - this appears to be what you have. A reference on the reverse side of the label such as ‘43-66’ means that the whisky was bottled in 1943. My guess then is that your bottle then is dated to be from 1975. To be sure, what kind of closure is on this bottle? A modern screw cap by chance?