Found in older Softubs with 3 jets triangulated on seat. Softub part # 9007900. Two bottom jets were typically spinner version, but all were interchangeable. Often clogged by small debris, and flow control left to right becomes restricted by sand and grit in jet body.
Removed by turning fully counter clockwise and pulling/prying out. Behind jet is found a black split ring threaded down on the flow valve which is sold nearby as ‘rebuild kit’.
The silicone fitted gasket that keeps flow concentrated through inner valve also regulates the air draw on these jets, so once failed air draw stops.