Here is a real treat! These are the reproduction front spring shackles sold in the catalogs right now.
I don't know what they are made of or how they are heat treated, but I do know how they perform. A friend of mine had these on his car.
The break was a clean crack indicating that it was very brittle. There was no elongation as it should have had if it was a correctly forged and malleable steel.
The best bet is just to avoid these reproductions and look for good originals or the old Bulldog brand replacements from the 1950s and 60s. The Bulldog shackles also have an appearance very close to the originals, and they are forged steel.