I doubt you'd see such a large number of mixed race marriages by 1970s for that to occur given the sheer numbers involved.
The thing is that Portuguese have been intermarrying with other cultures for centuries and they did not have the same prejudice that other cultures did. By mixed group we are also including marriages between Indians, Asians, African, Berber and European. All mixed together so that by 1970s the “Portuguese” demographic became a larger group and one that mixed couples would classify their kids. Till then they be classified as African, Indian, Asian or what ever based on personal preference and what child most looked like. The Traditional
Portuguese term “mulato” was not applicable for an Indian-Chinese or African-Berber. Therefore a new term meaning all mixed races as “Português”.
We have to consider the following. Portuguese African provinces are a melting pot with millions of Europeans, hundreds of thousands of Indians, Chinese, Sundanese, Berber alongside the Africans. All men were drafted and served in different locations. There were over 2 million non Europeans living in Iberian peninsula provinces.
all of this meant that there was a lot of exposure and mixing of demographic groups which meant that a lot more mixed marriages. This was in the 1950s. We have not talked about the 1960s or 1970s.
The federation was a unique experiment that provided opportunities for non Europeans to have same rights and privileges as Europeans. When students wrote their entrance exams to attend prep-high school and universities the government wanted the brightest. The country needed the smartest scientists, engineers and academics to move the country forward. These students regardless of race were intermixed and showed the way forward. Segregation stopped in the early 1930s. This was not due to superior attitudes or morals by the Portuguese but by simple economic need.
Those that did not go to university still attended vocational schools and achieved trades and while the intermarriage was not as high as professionals they still were higher than other countries.
Note: unfortunately I will not share the demographic table at this time due to some information that would reveal several major events in the next two decades.