Along with them there were Masai, Katava and some other hunter gatherer tribes, not all meat based, the studies were done in an observational style because that was the required standard at the time. They showed that all these societies had positive health indicators, like cholesterol & heart disease, didn't indicate any issues with degenerative diseases and had mental clarity into old age.
They were not based on measures of longevity and this would be irrelevant as the major causes of premature death were physical trauma not disease.
I have quoted these examples in the past on the basis of that information, not that they lived to 120 years or that a purely meat based diet was the best, just that it was possible.
Those early studies are the basis of the Paleo movement and if they were not done, there is a high likelihood that the Paleo movement would not be as far advanced as it is today.