Pages 503 & 504 of "The Parker Story" state that Parker changed the name from "Decarbonized" to "Plain Steel" as Remington was using the name "Decarbonized" on their newly introduced Model 1873 & 1875/1876 Hammer Lifter doubles. 889 Parker guns were made with the Decarbonized Steel barrel mark.
Decarbonized tensile strength is a bit more than pattern welded barrel steel and about the same as AISI 1018 Low Carbon (Mild) Steel. Winchester Standard Ordnance decarbonized steel was "cold rolled" to a slightly higher tensile strength.