These fairs were really popular at least back to the 1840's. When I was in graduate school one of the thematic areas in our class exhibit highlighted an ag fair from 1845 (we titled it Agricultural Fair, but from the discussion our fair also encompassed mechanical aspects). The only question left then is what the "D" stands for, and I lean with everyone else that thinks it's specific to the town where the fair was held.
Jim Sturgill
Manager of Education and External Relations
Mahoning Valley Historical Society