The S1 swivel is ideally used for 1/4″ – 5/16″ for Grade 30 or Grade 40 (G4) chain, the swivel will match or exceed the chains ultimate breaking strength. S2 swivel can also fit 5/16 chain to match the loads required for G70 chain. Since Grade 70 chain is uncommon most of the time S1 swivel is the appropriate choice for 5/16 chain. Oversizing the swivel is not recommended; as a bulky swivel can create fit issues, negatively affect anchor set and adds expense.
How come both S1 and S2 swivels fit 5/16 Chain, which one should I get?