50mm 2″ PVC Barrel Union Pipe Joiner for Pools & Spas. Used to connect 50mm pressure pipe, where pipe can still be removed. This is a heavy-duty union, not a cheap Bunnings one.
Note for non-plumbers: Please See This PVC Pressure Pipe Dimensions Info, as PVC Pressure Pipe uses “Nominal” names, and does not measure what it’s called.