You will need a special tool (MZ_UT105) to get to this o-ring, as it is above the diffuser. 3.53 mm thick x 20.22 mm inside diameter - 04081 EPDM.
Attention CA Residents: Prop 65 Warning
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.