Mike it is the high pressure relief valve in the rear of the
oil galley.
It blows off
oil back into the pan when the
oil pressure gets to high. I don't have a picture handy. I not sure but your service block should of had it too. There should bee a couple of allen plugs where they Machined the oil passeges from the rear to the front of the block. There's three around the cam plug and the two down the leftside. This is just from memory so you might want to check out some other resource or maybe someone can post a photo hear.
Tony