The S2 swivel is ideally used for 5/16″ – 3/8″ for Grade 30 or Grade 40 (G4) chain, the swivel will match or exceed the chains ultimate breaking strength. S3 swivel can also fit 3/8 chain to match the loads required for G70 chain. Since Grade 70 chain is uncommon most of the time S2 swivel is the appropriate choice for 3/8 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 S2 and S3 swivels fit 3/8 Chain, which one should I get?