My dad made this for me for a bike I’m building. My dad does amazingly good work.

The part is a spacer. It screws onto the hub of a rear wheel, and then a free-wheel screws onto the spacer. It makes the chain-line nice and straight for a single-speed without the need to re-dish the wheel. We designed it together, but my dad’s the guy who made it, and I’m not only indebted to him, but impressed by him.