The one at the back of the manifold is supposed to be blanked off. It is a throw back from the old days, or when the manifolds are fitted to other model cars that have economy meters fitted. The economy meter measures the amount of vaccum in the manifold, thus how much you have your right foot planted. If you want to fit a turbo to the engine, this is where you would fit the plumbing for the boost guage.
The blanked off pipe at the front, I've never come accross one there yet.
