Return nozzle fitting?

[Staceyleah22]

We bought one of those “pool chiller” sprinklers that is supposed to screw on your return jet. However the chiller threads seem to be different from the jet threads.

We bought a threaded collar at a hardware store but it doesn’t fit either. How can I figure out which type threads are on my returns?

 

[1poolman1]

They should be 1.5" pipe thread fittings.