This is a hook for a drop spindle. If you don't know what a drop spindle is, it is how roving was spun into yarn before the advent of the spinning wheel. Google knows more than I do. Basically, my wife needed a custom part. Here it is.

Fits on a post roughly 5.5mm in diameter (post was whittled so it doesn't have a uniform diameter).

Too small for infill, part is solid. Print laying flat for best results.

Scales up very well.