It depends on whether you count stamped ads or just printed ads, but
here are some that I believe there are fewer than 5 surviving examples of.
D310: Aldon Candy
E121: Service Candy, Shotwell, Kings, Henry Johnson (green)
R316: Douglas Park, Morton, Rialto, Community Theatre
1925-31 Exhibits: Marguerite's
W461: Playland
W514: Robinson Peanuts
W516-1-1: Sandow
W517: Piercy's
W554: Lucky Yo-Yo, Lukcy Deal, 40 Winners
1929 Exhibits Stars: Dixie Theater
W573: Cafe du Monde, Cream Nut, Kings
Here's the more extensive answer:
http://www.net54baseball.com/showthr...192333&page=11