Ibera Bike Seat Post Clamp Rack Mount Adapter 25-38mm with 25 kg Load - Bike Seat Mount Adapter for Rear Rack Installation - Universal Strap Mount - Replaces Braze-Ons, Compatible with Most Carriers

Mystery solved! It's the bolt from a cargo rack clamp to a seat post. Specifically the one in the link below. https://a.co/d/06Ca8LQh Sadly the square washer is gone, so I either need to disconnect it and take it to the hardware store or just order a new clamp. And we kinda have to use this one with the cargo rack on her bike.