I am restoring missing emissions equipment and have encountered a problem. I assume the biggest of the two holes on the back corner of the heads should be threaded and actually allow air to go somewhere. It is threaded, but it seems to go nowhere. It is right next to what looks like a quarter inch threaded hole. I expected a pipe plug, a bolt or an Allen headed plug. Instead, it looks like the threaded hole never went anywhere. The other heads that came from the same previous owner seem to have the same deal - no obvious hole or plug at the bottom of the threads. There is just iron or steel at the base of the deep bolt hole. I have uploaded a picture of a mirror aimed at back of the passenger corner of the passenger head so you can see too. Is there a plug that is threaded inside that could be removed with a BIG flat screwdriver bit?