I'm in the middle of restoring my intake actuators on my 89 na fc and I saw my replacement uim doesn't have this vacuum tee in it nor any other uims.

I think it may be a air injection hole of some sort but I can't find it anywhere since the casting for the mount doesn't even exist on other manifolds.



It's located right under where the bac was blocked off.

 

That is a sub-zero start assist valve. The S4 RX7 had sub-zero start assist, which would inject mixture of 90/10 percent antifreeze/water into the intake manifold to assist in cold weather starting. The sub-zero start assist system didn't work. Mazda later issued a TSB instructing dealers to remove/disconnect the sub-zero start assist system.

 

You can buy a block off plate from Atkins.