It looks like someone added an extra vacuum take-off port. Maybe to feed an instrument/gauge. Maybe to allow ad-hoc vacuum reading/testing from a hand-held gauge from time to time. Regardless, leaving it open will reduce the engine vacuum and impact the way the engine runs and will compromise the vacuum assist braking. Good vacuum is critical and air leaks like that are bad.
It looks like that same fitting has another port that runs to something else (I see a hose), so you should just blank off the open port. You will notice the engine runs WAY better and your brakes work better.
1968 Series IIa
1997 Defender SW (Original Owner - Sold)