I don't know about the 70s but a view of the 90s is that the children were there but things just weren't picked up enough of or that they were but a lot of teachers didn't want to know or didn't know how to help.
I got put in the remedial group for english in year 2 (1990) because my hand writing was bad. I was miles ahead in reading, my creative writing was brilliant (apparently - I'm not arrogant, honest ) yet because of my hand writing I was put in remedial group. I only have one memory of it and don't think it lasted for very long - it certainly didn't have any affect as my hand writing to this day is awful. I never could master holding a pen the 'correct' way despite being told off and people trying to force me to.
In addition to that my coordination was awful. I hated sports because I couldn't catch a ball, I couldn't do a head of heels or any other gymnastics, I certainly couldn't hit a rounders ball!
My social skills were lacking and still are. I have always had friends but these friendships don't come easily to me. Although my part in friendships didn't get any easier as I got older, I found that probably from about 18 people were much more accepting in general. At school outside of my friendship group people could be cruel - once I hit university people seemed to be more accepting. Or maybe not - maybe there is just the opportunity to meet a wider range of people as there were still the bitchy girls there!
I do have a full and successful career but one which requires a lot of communication with other people and I find it very difficult and tiring - it was a natural career for me to go into however because it involves a lot of writing and I was good at english so thought I should go into this.
Sorry, I'm going on but my point is that even in the 90s people missed a lot of symptoms or signs for things. From what I have said above and from a few other things that I don't find a hindrance but are just oddities about me, I think that I sit somewhere on the autism spectrum although high up (maybe the coordination difficulties are more dyspraxic though).
My DP has dyslexia as well. My MIL was told by the teacher he was stupid and that she should get him put into a 'special' school when he was in year 2 (this would be 1989). She wouldn't believe that he was stupid and they were lucky to have quite a bit of money and she was a SAHM. She taught him to read and write herself at home. Meanwhile they got him assessed for dyslexia and were told his IQ measured at 144 and that he was certainly not stupid - he was very bright but dyslexic. He ended up going private which for the majority of people is not an option. He achieved GCSEs, Alevels and a degree at good grades.
Moving forwards a close relative has been diagnosed with Aspergers and ADHD. He is in MS secondary and going through the statmenting process. The amount of ignorance that comes from a lot of the teachers including his Aspergers and ADHD being referred to as 'diseases' (said in a tone of disgust). It angers me that teachers even to this day seem to know so little - why can't it be made more of a priority to educate trainee teachers on SN, especially when a policy of inclusion is in place in this country!