Balboa Crescent air control valve usually used for controlling how much air comes from the jets. Stainless steel cap with dark grey handle. Do not confuse with this On/Off valve listed here. Remove cap to ensure it uses the 4 x tabs to lock on (see images).
Common on Spa Industries Spas such as Signature, Cyclone, Bullfrog, Designer, Leisurite and others.
AAIRCONCCG / 18900COALG