Baldwin D winding material

[Mike Spalding]

List,

Looked at a Baldwin D for "re-whatever" earlier this week.  The serial 
number dates to 1954.  One odd thing was that the mono-chord windings 
were copper, but the bi-chords were wound with iron.  Is this original?  
Is it unusual to have part copper and part iron?  Any and all comments 
regarding Baldwin D string scale welcome.

thanks

Mike