One fact, which y'all will appreciate, is that the TB influence in the King Ranch Herd is very, very strong. 97% "of the horse pedigrees analyzed for the King Ranch traced back to at least one thoroughbred ancestor."
Old Sorrell was at least 56% thoroughbred, and King Ranch breeders weren't afraid to keep the Thoroughbred influence in the early breeding program.
On the King Ranch, horses with Thoroughbred blood have been preferred roping horses because of their larger body size and speed, helpful for roping larger calves, cows or bulls on the 825,000 acre ranch. The Thoroughbred influence for King Ranch Quarter Horses born between 1941 and 1942 was determined to be 65.8% Thoroughbred and 34 percent registered Quarter Horse.
One assumes that the authors mean "post-registered" Quarter Horse, because the AQHA was only founded in the 1940's and the Klebergs were among the founders.