This is plastic cutting tool for DIY respirator. The shape matches the respirator from this site. https://www.sartor.cz/content/66-jak-usit-rousku (Sorry, the page is in Czech language)
The original pattern was a "S" size so I has stretched it to 115% to match it with my M/L size European head.
The best practise is to cut the fabric directly with roller cutter all around the edge of the tool. If you don't have a roller cutter, this tool can also serve as pattern for pen or marker with better grip on the fabric.
Fantastic tool for making "laces" for respirator is this thing https://www.thingiverse.com/thing:3880491