These are a 1/2" BSP thread, but the outside diameter has been reduced so that they fit in the drain cup as low as possible. Tricky to install you have to hold this nut in position inside the drain cup against the side, and then thread the barbed fitting into it. This means that when you are installing these the hose and barbed drain fitting both have to be removed and separated.
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.