M6 x 16 mm stainless steel with holes. This acts as the diffusor (vite di diffusore) in the La Marzocco group heads. Old version, to be phased out by La Marzocco, but there remain 10's of thousands.
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.