This is a plastic natch used to register pieces of plaster moulds for slip casting in ceramics.  I modelled it in Fusion 360 to be able to integrate them into my CAD on other projects rather than printing them out to use directly.

I've printed mother/case moulds with these integrated, and used the commercial natches to sit on them to pour working moulds.

Another very niche project, but I hope this can help people trying to make ceramic objects with CAD and 3D printing.