28mm cap diameter air cap for spa baths. 8 x slots. Chrome-plated plastic. Slightly domed top. No beveled edge. Similar, but not compatible with S&P spa bath air jet. Obsolete - Once stock is sold, these will no longer be available. There is no other replacement.
Air injector caps used on Lanark, Decina & Impact spa baths. Check measurements. These look similar to an older S&P air jet cap.