In the case of each other enumerated use permitted in NS, CS, PH or OLI Districts, one or two signs, affixed and parallel to the outer walls of the structure within which the permitted use is situated and facing upon either the principal street giving access to such structure or upon the parking lot appurtenant to such structure, provided that the aggregate area of each such sign shall not exceed one square foot for each linear foot of the building facing such street or such parking lot and devoted to the particular use or business housed therein, and provided, further, that no such sign shall exceed two feet in height or in length exceed 1/2 of the linear footage of wall appurtenant to such permitted use fronting on such street or parking lot. One of said two signs may be an illuminated sign, provided that it shall not be illuminated by flashing or intermittent lights, and no red, green, orange or yellow lights which, by reason of their location, might be confused with traffic lights shall be permitted in connection with any permitted use, and no fluttering or revolving devices, banners or yard signs shall be permitted. Where the building is set back more than 20 feet from the street line, one additional sign not exceeding four feet by five feet may be erected not nearer than 15 feet to the street line. The maximum height of the top of the sign shall not exceed 10 feet above grade.