What is this? What goes here?

[DLW]

Just pull the two bolts off and see if the hole goes all the way through if you can't tell by looking in the top of it. If its solid, you may be able to drill and tap for a sensor, this is where a lot of guys put their oil sensors, or use it to feed their turbo or oil cooler. Autometer also has an adapter you can put in place of the stock oil sensor in the back of the valley cover, then you can add your oil sensor their for your aftermarket gauge.