Sounds like the D0VE-C heads that you have are drilled with the smog pump passages. The belt-driven air pump forces air into the back of the heads and onward into the roof of the exhaust ports (in to the exhaust ports "smog bumps"). When the heads are installed onto the engine, the front of the heads have the plugs that you are asking about and the rear of the heads accept the air pump's tubular manifold.
If you will not be using the smog pump, then you can install the plugs into the rear passages as well so as to block them off. The expansion plug that we use is 43/64" (or about 17 mm) and is available from Clevite (through most american auto parts stores). Dont have the part number in front of me at the moment.