Waterway Button Air Injector. Miniature sized w/ 19mm diameter head. Uses 9mm hole. 1/4" straight barb suits 1/4" / 6mm tubing.
Can be used as an air injector (although 1/4" manifolds are not available, however 3/8"B x 1/4"B reducer adapters are) or commonly used as an airbleed return fitting on LA Spas and others to allow release of air from pumps and heater when filling (usually mounted in the filter housing).