New (to me) spa with no jets

[Jnet]

We bought the house last year and I spent most of the summer trying to find jets for the spa. There's a pic below of the opening. The overall face is about 4" in diameter. The internal thread is 2 1/2". I can't tell if the jet internal should thread directly into this fitting, (I can't find anything with that size thread) or if there's a threaded insert that goes in first. (can't find anything like that, either) The installation is about 25 years old and there's no text or indication of the manufacturer, although most of the original parts are Pentair. Anyone have any leads?

 

[jblizzle]

This might be the fittings like I have in my pool where a large "ball" with a hole in it (the jet) is inserted into the recessed area and then a ring is screwed into the threads you see to hold the ball in place. you can turn the ball to point in different directions.

 

[Jnet]

Thanks, jblizzle. I thought that at first, but the thread is bigger than a standard pool eyeball style jet, and it looks like it should have a spa-style jet that mixes air and water.