The head is drilled vertically from the deck up for the rocker feed but is not drilled all the way. The path takes a horizontal path. So the head was drilled horizontally from the side to crreate that path, then threaded and plugged. Fortunately for us GM did it this way because when the motor wears a bit after a bunch of miles, or the line behind the lifter cover gets clogged on the 54-57 motors, you can use that threaded port as an external oil feed to the rockers. Happy coincidence I guess!
