MH Pony - Manifold Question

[Palpatine]

I had to replace the manifold on my 1949 Massey Harris Pony. I noticed that both the old and new manifolds have a small threaded hole on the front of them, just above the carburetor. The old manifold had a pipe plug in the hole, so I put a plug in the new one as well. It looks like the hole is on the intake, and not the exhaust portion of the manifold. Can someone tell me what this hole is for?

Thanks.

 

[Dieseltech]

Hole is used for a vacuum gauge. Dad used the same thing on his MH 44 sometimes to provide a vacuum source to run the milkers too.

 

[Palpatine]

Interesting. So I assume it should stay plugged during normal operation?