DS Regulator for 1983 733i

I have a 1983 / 733i with hydraboost, I'm told that the DS regular may be defective. Can someone tell me how to determine if this device is defective. What does it do and how can I secure a replacement if needed (by the way what does DS mean)

I've also noticed that there appears from one of it's ports no vaccuum hose is present (any ideals?).
Again,
Thanks in advance for your help