Valve, lock ring and gasket to control air and water flow in older Softubs with three small therapy jets triangulated on seat.
Gasket typically degrades and fails allowing loss of jet pressure and no air draw through jet when air control lever is open. After removing jet by turning fully counter clockwise and prying/popping out, wall fitting must be unscrewed to access inner valve. Black split ring is easily removed by using needle nose pliers to unscrew by inserting in two slots in ring. Behind you will find valve and hopefully (part of ?) the gasket.
Kit comes with pre-lubed gasket and installs back in jet body, then split ring screws down to bottom out and hold valve firm. TIP: make sure ring seats down on valve with two access slots open through valve, as both jet prongs insert THROUGH these openings to control valve when jet face is grabbed and turned left/right.