Hi, Chiming in here....years ago I did restring an old Mehlin grand that had capo rods for all the treble wire sections....no agraffes in the mid-section. The iron plate was hollowed/or ground out/or probably "cast" appropriately to accept the size of these rods. The rods were of small diamerter and looked like nickel plated...but I'm no expert. They were really not grooved deeply either. I did polish them, and rotated them when I put them back in the piano, so there were no string marks/grooves for the restringing. I remember thinking what a good idea this was. For what its worth, Richard Adkins Coe College Music Dept.
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