Those date guesses are what I was afraid of. I also think the photo has a 1920s/30s look, but therin lies the difficulty.
This photo has an inscription on the back, saying it is "Dad's Mom" written by a daughter, so that would make the woman in the photo her grandmother.
Now issues are:
- The most likely grandmother died late 1929, registered as 70, actually older as in late 70s early 80s. Older than the woman in the photo.
- The other grandmother died 1917, registered as aged 66, actually 71. Not from the side of the family where the photos is supposed to have been taken.
- Can't be the a mistake for the dad's wife, as that was the writer's mother - she would surely have recognized her own mother.
- The dad was alive and around to identify who was in the photo. He did not die until 1978.