Stylus Air Force 2 Mk1 Spa Bath Air Injector Cap - Chrome - No Longer Available

NOT AVAILABLE - No replacement options

30.4mm diameter cap - Brass body that the cap clips over - outside diameter 21.5mm 
These air injectors were made by Hydrabaths - they discontinued manufacture in approx 2008. Stylus sold all remaining stock to us in approx 2010. No-one has any of the original parts remaining.

Replacement Options - unfortunately there are no direct replacement options available. 
You will need to have replacements caps or threaded fittings specially manufactured to suit. 
Replacing Cap only - it would need to be made from PVC with the diameter underneath perfectly sized to clip back onto the brass threaded body. 
Replacing cap / threaded brass fitting - unscrew the brass fitting from inside the bath. Take this to a fabrication company or fitter & turner and get them to make a new PVC or stainless steel replacement fitting with the same matching thread and a 6mm hole in the top for the air (or numerous small holes equalling the same surface area of a 6mm hole)