M6 x 10mm UNI 9327 - DIN 7984.
Most commonly used as the La Marzocco sight glass drain and group bleeder screw, or on the front of the E61 groups, but has other applications as well.
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.