It was used for more than one season, since there are two different helmets in those two photos. The top is the one season 'disco Giants' helmet. The second is the GIANTS logo with the white-red-white helmet stripe that was used for a few seasons before the white stripes were removed.
I understand what you're saying, but if a design is good, does it matter if it is a throwback? If it IS a good design and they had stayed with it, this wouldn't be an issue.
I'm ambivalent on this uniform. I'd prefer stripes on the pants and am not sure how I feel about the outline on the numbers.
If you were older, or saw older still baseball cards, you could have said it came from the Cleveland Indians or the Chicago Cubs, each of who used that logo in the past.