That finish is called Parkerized and you can do it yourself very cheaply. I started doing my own 20 years ago restoring Harley parts. Manganese and Zinc phospating developed by the Parker Chemical Co. way back in the day. All you have to do is soak the cleaned steel(glass beaded or wire brushed and degreased) in the heated solution for a bit and oil it after you pull it out.

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