This is a nut to secure a plughole drain for a sink. The thread on the plughole fitting is BSP 1+1/4 inch.

I made this for a portable handwashing basin that my Father's making.

It includes a couple of wings to make it easy to tighten up by hand because the standard plumbing fixtures are quite difficult to tighten up.

It also includes side cut outs to allow the side exits on the plughole body to drain and not hold water indefinitely.

It's quite fun to over-engineer something simple like this :)