Hi Mike,
Exhibits are great cards and for the most part very few were reprinted. You are pretty safe with the old exhibits from 1921-1938, a few were reprinted for shows in the 70'-80's but have advertising for shows on them so easy to tell. Jim Rowe did some 4 in 1 reprints in 1977 and they are easy to tell apart with a little studying. The main reprints are from the 1947-66 set and you can read more on them on the thread above, there are lots out there of these and pretty hard to tell them apart-
