Do you know what this is??

[Ieatcamaros]

I have never seen that before either. I don't see the threads that you are talking about. Obviously there are inside the hole a little deeper than we can see in the pic. Does it have threaded hole in it for a bolt the same size as the bolt in the other head? You can get a slug made to look like the other one and just bolt it on if that's the case. I think that it is most likely egr/smog related. Are there any "ports" on the back of the heads?