Just noticed on a spare front axle I have, it came from a chevy P-30 chassis, 80's probably.
When it was removed the sway bar (about 1.25" diameter) was cut with an abrasive cutoff saw.. then it's been sitting outside for months..
Now that it has some rust I can clearly see about 1/8" around the edge that appears different from the core.
Not sure what that means, but for sure the bar is not the same throughout, some sort of treatment to the outside.
Grigg