Hi Chris, I believe that's a bronze tuning pin bushing. They're used to shore up a loose tuning pin in the pin block until someone like you comes along and replaces the pin block. By the looks of things it's at ground zero.
Is that an emery cloth bushing next to it? Old school field repairs. I was taught these methods in1971.
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Larry Fisher
Owner, Chief Grunt, Head Hosehead
Vancouver WA
360-256-2999
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Original Message:
Sent: 03-07-2017 12:11
From: Chris Chernobieff
Subject: Fixing a Hole
Found this today. Ouch! A copper plug of some kind.
http://chernobieffpiano.com/
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ChrisChernobieff
Chernobieff Piano and Harpsichord Mfg.
Lenoir City TN
865-986-7720
chrisppff@gmail.com
www.facebook.com/ChernobieffPianoandHarpsichordMFG