In the early 1970s my father, who was a Scottish neurosurgeon, worked with Korean war vets in Bethesda Naval Hospital researching brain surgery techniques used during the conflict. He was presented with this Bulova Accutron Date watch at the end of his residency. I had the watch fully overhauled - including a new glass - in November 2014 by Swisstec, a division of Swiss Time Services Ltd, at a cost of £215. It is still in full working order. As you can see from the attached photos it is dated 1970 (N0) and has a hallmark of 40SAD on the side of the watch case.
I think the SAD might in fact be SAO which I think is an English stamp. The 40 I am assuming means 40 microns.
I've had a deep dive into the linebooks and complete guide of Accutron watches, but was unable to find a dial match to determine the model number/variant.
However with that said I agree with Jim in that the watch is a 1969 Bulova Accutron Calander.