what you read

and I have seen matching exhaust manifolds with this hole and some which doesn't have it

Those are for the smog pump. They can be tapped and plugged with Allen set screws.

Correct.They are for emissions.Plugged lots of them.Rocky

but why emissions... what they do on how they work ?

Starting in the mid 70's there was a smog pump, belt driven, that would pump air into the exhaust via that hole that would combine with unburied fuel that would then burn in the exhaust.

You don't want it, you don't need it. Plug the holes.

A I R air injection reaction An air pump, usualy sliding vane design, pumps air into the exhaust passages which help complete combustion of the unburned hydrocarbons and CO. They first appeared on chevies in about 1968 and were on some engines up through the beginning of three way catalytic converters. In order to get a three way converter to work the A/F ratio had to be controlled within a very narrow window and the AIR injection didn't fit the control scheme.
Some AIR systems ran tubes into the exhaust port. Some used a small hole in the exhaust port which was fed through an isolated port system in either the exhaust manifold or the intake. Pontiac and possibly other engines used the negative pulses in the exhaust ports to draw in air past reed valves The system you are mentioning cost virtually no horsepower but the belt for the pump was often removed in the name of more power. It was something to be done by the owner to improve performance, although the results were inconclusive.

The heads have the hole that runs straight from the exterior into the exhaust passage and there was a small bump to help mix the air. This hole is straight and usually has a lot of carbon in it. I was going to use older exhaust manifolds and so I plugged the holes. My plan was to use 1/4-20 screws so after getting the 1//4-20 tap I got the drill bit necessary ahd drilled the holes. I thin threaded the holes. The cast iron of the heads had some hard spots and I managed to break off a tap in one hole. I found eight short bolts and threaded themn into the holes, then cut off the bolt heads with a hacksaw. After I ccleaned the holes by drilling I could have also used JB Weld to plug the holes but the little bolts were free and were so low quality that I didn't want to use them anywhere else. You could probably plug them with oak pegs or RTV or many other things.

R.

I was aware of the air pump setup ( which in fact I think is the reason on BB the steering pump was installed lower into the engine bay and water pump housing changed the lower outlet to the passanger side since 73 ), but never analized where or how was making the interaction. I recall they were conected to the carb, and not related to the exhaust system

now wondering about this system on BB, since they don't have a similar setup on heads/exhaust manifolds, at least I have noticed

Hey Nacho, Not sure what you mean by BB and in relation to the heads you posted??

Whether they are present depends on when the heads were made and for what engine. See for example: 1973 Chrysler Clean Air MTSC, Air Injection

As far as the purpose, Pacnorth has basically covered it. The air injection into hot exhaust helped consume unburned hydrocarbons and also reacted with CO.
For Chrysler products, cats came later.

You're lucky. On my first engine install, I didn't notice them until after firing it up. The guys in the exhaust shop pointed them out. Not much fun tapping and installing the setscrews afterwords but it was a learning experience.

head is of course SB, not mine but a friend

wondering about why SBs got this an not BB ?

Some did.

by the mid 70s went those showed up on the small blocks the BB were on their way out...stopped in 1978, they were only used in pickup trucks and RV which were allowed to run dirtier based on small laws.