Well my guess is that the pin is in shear loading and the rest of the shackle is under tension. that would account for the difference in size. As for the load rating, my understanding is that in the "industrial rigging" area a safety factor of 5 to 1 is used. So with that the 5/8 shackle should have a breaking load of 5 times the rated WLL (working load limit). If the WLL is not stamped into the shackle (there is lots of stuff that only says "CHINA" on it) then its anybody's guess as to what it will take.
Carl
S/V Moonflower of Moab
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