That's my mistake. You are correct, sir, not the serial number. Learn something new! These would be made to spec but not traceable shackles in that case.
Looking at the standard for shackles, ASME B30-26 states:
"Each shackle pin shall have durable markings by the manufacturer to show
(a) name or trademark of manufacturer(b) grade, material type, or load rating"
These are standard screw pin shackles:
https://www.thecrosbygroup.com/prod.../crosby-209-carbon-screw-pin-anchor-shackles/
It's marked X0CG6 so I think it breaks down to:
X0 - unsure, may be as you say a part number, process, alloy
CG - Crosby Group
6 - I think this means grade is 6, e.g. a 4.75 ton 3/4" shackle