When commercial non-corrosive smaller arms primers appeared on the market in the early 1930's, manufacturers played up the non-corrosive (rustfree) feature. It really was a major improvement over what had been available for the previous 80 of 100 years. With smokeless powder and non-chlorate primers, NO bore rusting after firing.
Remington-UMC called theirs "Rust-Free".
Winchester (later became Winchester-Western during the Great Depression) used the brand name "Staynless" to denote that their ammunition did not cause bore rusting and corrosion. This advertising was continued well into the 1950's and early 1960's.
HTH
John
