FWIW, have here my original bought in 1976 packets of removed RS66 Swing Bass 105,080,065,045 strings and still have same era strings on my P bass and Rick 4001 to this day ( spend my money on basses not strings...

).
Stainless steel has many variants, Martenstitic and Austenitic. From completley non magnetic to weakly magnetic attraction. Lost on me how a pure Stainless steel string can work in a disturbed magnetic field PUP's like we have in our basses. Other than eddy current induced magnetic field in the SS component. Tested the 1976 strings here and the outer core seems as magnetic as the inner core. Not weakly magnetic attraction at all. This makes sense to me for PUP output signal will be proportional to disturbed magnetic field from strongly magnetic material not almost diamagnetic SS would produce little signal output comapred to the Steel ( might be nickel steel as KPAX indicates, i do not know for certain ).
So KPAX has merit in his assertions given that i have strings of the era here on hand to test. No 1960's strings here to test.
EDIT: Never has KPAX asserted the Rotosound RS66 longscale Swing Bass strings from the 1970's to be nickel plated or pure nickel metal. He states Nickel steel ( alloy of iron ) just like the soft frets on my 1975 Fender P Bass which the RS66 swing bass chew out ie steel/nickel alloy as i understand. It and the Rick 4001 are the only basses i own being effected by the RS66's to a noticable degree of hollowed out frets. Perhaps to be correct in this discussion, we need to define "Stainless Steel" as the variaty is vast as to SS alloys.
Hope that helps, John ( Played Bass since 1975 )