Pump union, with o-ring, for 1.5" pipe. This union works with several brands of pump, notably Waterway and ITT/HydroAir. The thread is of the asymmetric "buttress" type (also known by the initials MBT and FBT for the male and female versions).
Inside diameter of the thread: 75 mm
Outside diameter of the corresponding male thread on the pump: 78 mm
Connects to 1.5" pipe (outside diameter: 48 mm)