Brake Booster outgoing hose question
 

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.

That is one way vacuum check valve(no adjustment need) hope this help.