Generally shackles come in 2 grades:

• Industrial Grade (Type IV A)
• Specialty Alloy Shackles (Type IV B)

Generally if you are trying to match the strength of your chain you can use these rules:

• If sizing for BBB or Proof coil chain use Industrial Grade (Type IV A) shackle, one size bigger than your chain
• If sizing for HT (G4) chain use Specialty Alloy Shackles (Type IV B), one size bigger than your chain
• If sizing for stainless steel chain use a forged SS shackle that is one size bigger than your chain.
• If sizing for duplex chain use a duplex shackle one size bigger than you chain