"Decarbonized Steel" is "Bessemer process homogenous wrought iron" and was sourced from Remington Arms. Pages 503 & 504 of
The Parker Story state that Parker changed the name to "Plain Steel" as Remington was using the name "Decarbonized" on their newly introduced double gun.
1869 Price List
"Best Gun Iron" were the musket barrels used on "Charles Parker Makers" guns
1866 Price List
This question comes up frequently and maybe could be added to the FAQs?