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Edit: Here is a blurb I found on Wikipedia

Mangalloy is made by alloying steel, containing 0.8 to 1.25% carbon, with 11 to 15% manganese.[1] Mangalloy is a unique non-magnetic steel with extreme...

Manganese is used in the production of Stainless Steel. I don't know if this is an antique but if it is maybe this is what they were calling it before they changed the name to stainless steel.