are going to be found on all of the cylinders, at least every cylinder I have seen. The #3 cylinder is the one that is easiest to use for manifold pressure indications. The other three are available for primer installation.
My engine was a low time factory reman from a certified plane. It came with the primer system plumbed to each cylinder. There is a second port in the top of each cylinder so manifold pressure can be taken from any one of them.
