Rubber washer for air extractor units in old double trap syphonic / siphonic toilets.
This is the universal type, which fits a few different air extractor / aspirator / airex / syphon bomb units, you might need to make the hole in the middle a little bit bigger.
Here is a diagram showing where the washer is fitted. It's labeled the rubber plumb washer.
![Diagram showing syphon air extractor washer position Diagram showing syphon air extractor washer position](https://www.stevensonplumbing.co.uk/images/source/Plumbing/WC_components/Airex_fitment_2.jpg)