The slot was origionally cut with a horizontal milling machine cutter. This could have been done either on a horizontal mill or a vertical mill at the factory. The good news for you is the slot is a low precision aspect of the part and you can be cut by hand. If the cutter is a steel horizontal cutter it must turn slowly with a machine such as a Foredom Flex Shaft machine. It can also be cut with a die grinder or Dremel tool but due to the speed of the grinder an abrasive tool must be used.