Art, if it has no collector value, might as well just blue it if it bothers you and shoot the hell out of it. It would make a nice "car" gun in that it would not bother "too" much if your car ever got broken in to.
That said, if the slot is not very deep, I might consider just machining that whole area and just end up with two vertical or slanted edges instead of the more obvious looking round ended "slot" prior to blueing.
You could also just carefully cut a piece of grip tape and stick it on there.... it would just look like what most of us IDPA guys do with grip tape in strategic places on our competition guns.
