These eye bolt adaptors convert 16mm to 9/16" allowing you to use rear leaf springs from the previous generation (1988-1998) Chevy/GMC pickups (1/2 ton 3/4 ton and 1 ton) on the newer generation trucks which come with lighter capacity leaf springs and are typically a lot more expensive. It's a great solution and allows you to save money and have a nice heavy suspension.
This kit gives you all 4 adaptors which is enough to do both rear leaf springs.