[snopercod]

OK, it took me a while, but I found it: The "only approved manufacturer of armor plate in North America".

Clifton Steel

See, it's not the steel so much, as the required hardening and tempering processes.

Thank me.

[OXENinFLA]

I don't think that's it.

Durbin referred to a mill in Pennsylvania.

What you just posted is a "manufacturer" not a foundry.

Clifton Steel works with the steel but it's doesn't look like they found* the steel.

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*found
1. To melt (metal) and pour into a mold.
2. To make (objects) by pouring molten material into a mold.

[Hermann the Cherusker]

only approved manufacturer of armor plate in North America

There is a difference between being "approved" and being "capable".