This is for the Bemis NextStep2 toilet seat with soft close hinge. It may fit other Bemis toilet seats. It seems these hinges are prone to breaking. This is what a broken OEM hinge looks like:

You'll want to use supports for the inside as shown (organic works great for this). My toddler has broken 2 OEM hinges and one printed replacement already, so I'd suggest cranking up the walls and infill.