> As someone who grew up in that era, I can say all the family photos have not been edited, nor would the majority of people who owned a camera back then spend the time or money to do so.
Whoever developed your photos would've adjusted the brightness and white balance based on what looked right to them if nothing else. Send the same negatives into two shops and you'd get different-looking pictures back.
It was an automatic process and colours were consistent. I find that reading theories about what might have happened 20 years ago is becoming pretty annoying. I shot some kind of Fujifilm mostly and if it was under/overexposed that was like your problem.
That the editing/retouching changes small parts of the image and materially alters it to show or leave out stuff that was not in front of the lens and that changing whitebalance and such affects the whole image uniformly.
Whoever developed your photos would've adjusted the brightness and white balance based on what looked right to them if nothing else. Send the same negatives into two shops and you'd get different-looking pictures back.