From what I searched, it seems to be the toe of a random person who donates it. Not quite the same allure but still a cool idea.

https://www.atlasobscura.com/places/sourtoe-cocktail

Toes have been stolen or swallowed (yup) over the years and replacements have been donated. There's now a hefty fine for anyone who wants to swallow the toe.

Thanks for the reply... I googled. Amazing tradition. Not sure I'd like to participate ;)