The shackles are natural/tumbled . The NOS service shackles are also bare steel.
Natural, aka untreated, steel rusts. Original steel shackles are rusty (that's unfortunately true). My solution is to bead blast and phosphate them - lightly. It's very close in appearance to a part stamped from cold rolled steel (CRS) bar, and oiled down as part of the process, to keep the stamping die from failing.
It's your choice, base steel, or a light phosphate and oil finish.
Jim