Having just installed strap buttons on my classical I can say that the felt washer compresses and the result is a tight fit with no play in the strap button. I can see the felt washer serving two purposes. First it fills in the voids that will be there when attaching a flat strap button to a slightly curved wooden surface. Second, I suspect it helps keep the bottom of the metal button from scratching the finish. (yes, I know there is still a hole there if the button is removed
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