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...
This part is a LA Spa Gray Swirl 3 15/16" Spa Buttress Style Diverter Cap only. This is a Buttress Cap. You need to inspect your old cap and see if your cap has the letters LA or the word...