Not only a sight glass o-ring: where other manufactures use copper or paper seals, Nuova Simonelli use this o-ring to seal some metal fittings on their equipment, in addition to their sight glasses. For another example, the ECM (and others) use this as a seal for their steam wand spring seat.
ONLY THE OPTIMA MODEL USES THE S_48 AS A SIGHT GLASS GASKET! All other Simonelli models use this o-ring.
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.