I too know of some Angus faithful that will insist that every Angus animal in the Angus herdbook is pure Angus. However, I have my doubts. Though I feel that most Angus producers are honest, I also know some who are known to bend the rules to their favor, and could be persuaded by the precious dollar to do some less than honest things.
In the 60's the vast majority of Angus cattle in the US were small or relatively small framed. I find it interesting that in less than a ten years the Angus breed had significantly increased the frame size of a large percentage of the Angus cattle. Think about it. If in 1967 many Angus breeders decided that they needed to increase the frame size of their cattle and bred a high percentage of their cows to one of the larger framed Angus bulls of that day (which still wasn't all that tall) the first daughters of this breeding would be born in '68 and wouldn't have their first calf until '70, which would be the second generation of this focus toward larger cattle. Those second generation heifers wouln't have their first calves (the third generation) until '72. The point I am trying to make is that it takes a long time, due to the generation intervals of cattle, to make a significant change in phenotype. Maybe I'm just being skeptical but I have a hard time believing that the Angus breed was able to change so much in the 70's and 80's without the influence of some dairy or Continental influence.
A good illustration of the changes in physical changes in cattle over time can be found at a:
https://www.msu.edu/~ritchieh/historica ... etype.html
To add to my skepticism, over the years I have had different commercial producers comment that they occasionally get a horned or flowered calf out of their registered Angus bulls...yes, bulls registered with the American Angus Association. I apologize if this sounds like Angus bashing, because it isn't. In my opinion, Angus probably do more things right than any other breed, however I question if every Angus breeder in North America was able to stand firm to his commitment to breed purebred Angus cattle, when it would have been soooo easy to just add a bit of Brown Swiss, Holstein, Limousin, or Chi to some cows in order to get a jump ahead of the competition in the frame race of the 70's and 80's.