Its the main air feed where the air line would go when a coupler is on the little end tried machine mart and no joy so if anyone has a link to one would be great
I was thinking looking at the first pic, it would have been nice to see a ruler or tape measure along side the fitting. Then you put a bottle cap next to it but WHAT SIZE bottle
Note BSP threads are not as directly measured namley a 1/4 BSP (outside, male thread) will measure up as aprox 1/2".