This is Harken's small Stainless Steel U-Adaptor. If you are looking for an adaptor that will enable you to attach blocks with 3/8 in (10mm) posts to swivel bases, other blocks with 3/8 in (10mm) posts, tangs, or chainplates, you are in luck! This gem is ﾽ in center-to-center on the 3/16 in clevis pins it is 11/16 in wide.
Harken's midrange (large) adapter, H1598, is also ﾽ in center-to-center on 5/16 in clevis pins and it is 7/8 in wide. No spare parts kit should be without these adaptors.