97 mm pump union with o-ring.   This union works with several brands of pump, notably Waterway and ITT/HydroAir.  The thread of the asymmetric "buttress" type (abbreviated as MBT and FBT).

This thread size is sometimes referred to as 2.5" FBT (even though it's much larger than 2.5 inches - please call us if you can explain this!), but the union provides a female socket for 2-inch pipe.

Dimensions
Internal thread diameter : 90 mm
External diameter of the corresponding male thread on the pump : 94 mm
Connects to 2" pipe (external diameter of 60.3 mm)

Part numbers
Waterway : 400-5990