You can apply some common sense though. Women opening bank accounts was not some fringe thing only available to a few in 1975. It was the norm. In a city, the vast majority of banks would be open to women. Maybe there was one old fuddy duddy bank that refused to do it. But the vast majority would.
This is how anti-discrimination laws always work. The only way an anti-discrimination law can pass is if the vast majority of the population is already onboard with it. Laws tend to be passed banning discrimination when the tolerant majority gets tired of putting up with the bullshit of a bigoted minority. Until that threshold is reached, the standard is always "let people and companies decide on their own."
Jim Crow was defeated when the vast majority of the US population had come to the point where they believed racial discrimination was wrong. It was the rest of the society collectively telling white people in south "we're tired of your shit."
If most women in the US could not open a bank account in 1975, then the vast, vast majority of banks must not have been offering them accounts. The only way that would happen is if the vast majority of the population opposed women having bank accounts. And if that was the case, there would have never been the political will necessary to pass an anti-discrimination law. Anti-discrimination laws tend to only be passed when they're banning forms of discrimination the majority already opposes.