This part is an Air Injector Connector, HydroAir, 3/4"spigot x 3/8"barb.
This part receives a 3/4" Flex pipe in the spigot side that would measure about 1" Outside diameter and then a 3/8" Inside...
Multibody air injector in white made by Waterway. This air injector requires a .60" hole size and comes with a small gasket , retaining nut and three different barb adapters to suit multiple plumbing...