Do you know what this is??

[bmxmon]

yeah, sorry about the pics not showing the threads. There are no other holes on the outside of the block, other than the one I am trying to figure out. Getting a slug made sounds good, but where do i go to do that?? I'm pretty sure that they are smog related also, because of those humps in the exhaust ports, the holes connect (the ones that go through those humps) to this odd thing on the outside of my heads, without a plug I would have a major exhaust leak. Thanks for the help.