Pianotech

I've been working on a Cable-Nelson upright grand that I believe is from 1907 (I found someone wrote that on the piano). Was a different metal used at that time besides copper for windings? The bass strings look silvery where the hammers have struck them frequently. I replaced a string (B flat 2), but it would only be comfortably an octave below where it should be when trying to tune or get it close to being in tune. When I would try to get the two strings for them in unison, the replaced string (bright copper in the photo) would have to be tightened far more than its neighboring string (the darker string to the left). 

I have seen new pianos with silvery (zinc plated iron) wrapped bass strings or sections of bass strings.  Some manufacturers must have found that the iron wrappings had the sound quality that they wanted and they used it.  It was probably discontinued because it looked  "cheap".

Many inexpensive smaller pianos from the 50s through the 70s have copper plated iron wrapped bass sections.

I have found that iron wrapped bass strings didn't  seem to hold up as well and didn't "turn" or revive as well as copper wrapped strings.

General practice would be to replace both strings of a bi-chord even when only one of them is broken.  This is because the new one will always be a slightly different mass then the one you are "matching" it to, which will prevent you from having a clean unison.  I would assume the string you had duplicated has an error in it.  Even though it is a different material it should be "in the same ball park" because the string should be made specifically for that individual note for that specific piano for that particular partial and fundamental.  I'm highly surprised only one string is broken/replaced.  Actually I'm surprised it's tuneable at all.  I'd say count your many blessings with this one and good luck 😎.