it came out of my tally book, cheet sheet. these are the standard load ratings of shackles used in the oilfield for those sizes, but your correct that some manufactures would have their own rating stamped onto the shackles and some yeld a higher rating WLL. this is a basic guide line and other manufactures would either rate their shackles the same or higher if better alloy is used. I believe ICC 7/8" shackle is rated at 14500lbs and CM big orange rates their 7/8" shackle at 18700lbs.