Those are Danco faucet washers, stock number 61805B. I couldn't find 'em at my local Ace, but they were at Menards. They're rubber, so they stretch over the button, and once they're there, they're not coming off. 67 cents.
As you see, I have 'em on Tila, my blonde guitar, but I might swap them with the Schaller strap locks on my #1.