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ALL Prometheus springs carry the naming convention of "MSXXXSP". WGC just doesn't include it in their product description/name. I suppose they figure the customer would know what they were talking about. When you get the item, the package says "MSXXXSP".
Typically, all black coloured springs are oil-quenched (such as the Systema M120). Springs that are coloured otherwise are either anodized, plated/coated, or as is the case of the Systema M120S, are made of an entirely different material (Stainless Steel as you correctly guessed for the M120S.)
I find the M120S to be a hard hitting, but very very consistent spring. One of the most consistent springs for that velocity range, if that's something you're looking for.
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