RD 400 cylinder ?

I have a 78 RD 400 which I am rebuliding.

Near the top of the Cylinder wall just above the exhaust port approx. 10mm is a hole maybe 3mm in Dia. which travels down into the exhaust port.

Is this there to de-tune the engine a little?
Could I gain some extra snap by somehow blocking this hole or does it aid in scavenging by creating some sort of added flow to the exhaust gasses?