Well, a picture is worth a thousand words, but I don't own a digital camera.... If you look at an early center oiler block, in the area where the
oil filter adapter bolts to the side of the block, there is a small "extra" hole that gets covered up by the adapter. Not all blocks have them but most do. It is pretty small, about 1/8" or so. It is just a straight hole from the outside of the block through to the crankcase. If you do not plug it, the
oil pressure will likely rupture the paper
oil filter adapter gasket and your oil pressure will just shoot right back through to the crank case. No oil will go into the oiling system. I only had to see this happen one time, and I decided that I would plug all my blocks just to be safe. I have no idea why the hole is there, but it sure can be an engine killer.