Very common air control. Top part can be unclipped to replace cap. Have been various shades of gray over the years. Common air venturi control used on a number of Australian spas such as Signature, Spa Industries, SCS, Lanark, Leisurite, Designer, Bullfrog and many more.
AAIRCONDCG. Also sold as Davey Q3790GY
Hole Size: 41mm
Pipe Size: 25mm / 1" socket
The cap's clips may break off over time. New fittings will need the caps removed carefully, and then clipped back onto the old fittings.
If water is coming up from the cap, there is usually an issue with the spa jets, such as too many jets closed, or a blockage at the jet. These work by venturi air suction.