AN EXPLANATION OF THE RELATIONSHIP BETWEEN 3RD PORT DESIGN AND 9 PORT McCulloch engines have 3 intake or transfer ports on each side of the block giving a total of 6 intake ports in standard configuration. The extra 3 ports are gained by raising the level of the third ports then lowering the 3rd port wall under the intake manifold.

This photo shows another view of the ports through the exhaust port This example is a NOS factory MC90

This photo shows the factory casting line of the lowered 3rd port wall on the MC90

The only relationship between the 3rd port and the 9 ports is that both are a part of the induction and transfer systems. The 3rd port serves to increase the amount of the charge in the crankcase through the velocity of the air moving through the carburetor. In theory, the idea is to gain atmospheric pressure in the crankcase at the termination of the induction portion of the cycle. In practice it often doesn't quite work out. The nine-ports become active on the transfer portion of the cycle and serve to increase the volume of fuel transferred from the crankcase to the compression/firing chamber. Since you can always add larger carbs or multiple carbs there is an increase in the level of crankcase charging and the 3rd port system is not needed. It is obvious that transfer efficiency is now more important  so 9 ports provides more opportunity for power than the 3rd port arrangement.

This photo shows how the factory raised the 3rd ports up to match the other 6 intake ports This example is a NOS factory MC90