proprietary name for a brand of heavy automobile trucks, named for brothers John M., Augustus F., and William C. Mack, who established Mack Brothers Company, N.Y., N.Y., in 1902. Their trucks have been formally known as Mack Trucks from 1910.
Figurative sense "chief element of a system" is from 1640s. Pennsylvania was called the Keystone State because of its position (geographical and political) in the original American confederation, occupying the middle (7th) place in the "arch" of states along the Atlantic, between eastern states and southern ones. Keystone cops were the bumbling police in the slapstick silent movies produced by Keystone Studios, formed in 1912 in Edendale, Calif., by Canadian-born U.S. film director Mack Sennett (1884-1960).