Parkerizing or Phosphating is a Metal Finish that really gained in popularity during WW2 when the US Government was looking to replace the typical blued finish on most small arms with a Rust Resistant and Anti Reflective Finish that would be both Durable and Abrasion Resistant and hold up in all weather extremes, for this they chose Parkerizing or Phosphating.
The Parkerizing technique is a Phosphate etching process that produces a Hard Matte or Dull finish that is both very Durable and Anti Reflective and with excellent oil holding properties.