the hole ur refering to just has an 8mm hex grub screw in it. its the one in the pic with the rag in it. the one to the right of it is were the oil presure swithch goes. to the right of that you can just see another hole like the first one, with the grubscrew in place. these are for cleaning block. there for gun barrel brush. plug it with a cut down bolt, with a copper washer n forget it.