The burnishing action of a jewelers brass brush used with water
and detergent may be the easiest way to polish this harde to get
to area. You can also "work " off of the edge of a firm buff.
There is a type of buff called knife edge. As you might guess,
this has a very narrow edge that seems easier to work with some
areas. If nothing else, it's easier to see exactly where the buff
is on the metal. Keep trying, you'll get it.