Yep, that is the air injection port for one of the emission controls. I have a head just like that sitting on my parts shelf.

You will absolutly want to block the port or you will get exhaust coming out where you don't want it...

The only concern I have is the fact that you will have hot exhaust traversing back and forth between the 4 exhaust ports inside this passage. May increase the heat load on the cooling system some...

Each exhaust port has a ~8mm hole that connects to the passage that is visble in the pictures.

Back in the early 80's a friend of mine was tweaking on a Toyota 20R or 22R motor. It had similar air injection ports as the ones in the A14 head. Blocking the air inlet kept the exhause under controll but the air ports in the exhaust port pockets caused the exhaust to have a strange whistle to it. His fix was to have the air injection ports in the exhaust ports welded shut.

Gary