This is Harken's large Stainless Steel U-Adaptor, aka Harken's Midrange U-Adaptor. If you are looking for an adaptor that will enable you to attach blocks with 9/16 in (14mm) posts to swivel bases, other blocks with 9/16 in (14mm) posts, tangs, or chainplates, you are in luck! This gem is ﾽ in center-to-center on 5/16 in clevis pins and it is 7/8 in wide. Harken's small adapter is ﾽ in center-to-center on the 3/16 in clevis pins it is 11/16 in wide.
No spare parts kit should be without these adaptors.