This is an early (circa 2006) Dyson vacuum cleaner style pre-motor filter for the barrel Dyson DC05 and the barrel style Dyson DC08 and the Dyson DC08T as well as the upright DC14 (Clutchless) and the upright DC15 and the barrel style Dyson DC19.
It's overall diameter is 153mm and 29mm thick.
It's partnered with Post-Motor filter