La Marzocco Drain Cup Barbed Fitting Fixing Nut
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.
La Marzocco Part number L037/D