steel material question

All,
I am looking at a drawing that specifies the material as: Steel, Class C6, ASTM AS400 Or Steel CSTG, Class 150-125or 175-145.
Any idea what Steel Class C6 is? I can't find it anywhere. I think the CSTG is for casting. The part could be machined either from solid or a casting.
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The AS400 referrs to iron-chromium alloys without added nickel.
--
Anthony

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