For reference, have '77 450 sEL 4.5 L (W116).

As you know, there's a big, clear vacuum hose coming off of Brake Booster. About halfway down this large line, there's a joint and a large circular plastic joint or valve there in the line. There appear to be some markings on the outer ring of this plastic valve/joint.

Does anyone know what this is? What the function is? What the markings mean? And how this should be adjusted?

Your insights would be most appreciated!!
Thanks.