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ESS-M3P20-A
(S2)
Zn Phosphate 72

S2 is a Zinc Phosphate and Oil treatment.

Found this paragraph in on a metal coating website which seems to indicate they can color zinc phosphate for a nice black finish.

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Zinc phosphating can extend the shelf life of your products in corrosive environments. By combining our zinc phosphate treatment with rust-inhibiting oils, wax, and other organic coatings, we can help your product withstand harsh conditions and up to 168 hours of continued salt spray. Jackson Tumble Finish has the ability to coat heavy, medium, and fine grain zinc phosphate, along with black phosphate and other color options.
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Unfortunately at this point Caswell may be the only reliable source .... next time I buy some manganese or "Parkerizing" solution I'm going to try a gun smith supply store. Typical firearm Parkerizing is usually very close to black.