This is an end cap for a curtain rail.

I don't know the name of the kind of curtain rail it's for, but they're common in New Zealand and probably other countries.

A small piece of rail is shown in the photos. The rail is 25mm x 12mm.

I made this entirely from scratch in OnShape by measuring one I have with a broken loop on the bottom.

I've printed in transparent Elegoo PETG Rapid and it seems plenty strong enough, but it feels like the original I measured is made of ABS. I don't have any ABS or the ability to print it but that might be the go if you can, or maybe nylon. I wouldn't recommend trying this with PLA.