1.50 mm "coarse" thread pitch. This varies from older models, from about 1978 to 1996 that had the 1.25mm "fine" thread boiler and flange. The 1.25 mm flange is out of production, so it would require a new boiler to match these newer flanges.
Some customers have found a strap wrench or similar tools for oil filter removal are useful in helping to loosen this flange from the boiler.
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.