Need Appropriate Catalog Information

Am not succeeding in locating adequate information on this envelope. It is a Bicentennial Era - The American Craftsman 13c and has an exhibition stamp for "Cornpex 77 Bloomington, IL 61701 Philatelic Exhibition and Bourse". Searches have brought up others without the exhibition stamp and variables of "Cornpex" as "Compex" and "Complex" Nothing for this specifically. Can anyone help me properly identify this? It is marked as a U575. Does the exhibition mark change anything?

Would appreciate any information.

The cover is US Scott U575, issued August 6, 1976 (first day city Hancock, Mass). A used entire has a 2011 catalog value of $.20. The cancellation places it in the "event cover" realm but I can't tell you if it adds a premium. This Colorano silk cache from that exhibition is selling for about $4.40.

I like that one! I didn't even know about "event covers" - learning here :) Thanks T, appreciate the feedback.

I never seen that one. Cool cancel also!