From Here:
http://dd-b.net/dd-b/SnapshotAlbum/data/1976/09000-Midamericon/Quoting David Dyer Bennett:
"Midamericon, the 1976 Worldcon
Midamericon was at the Radisson Muehlebach hotel in Kansas City, MO.
It was the biggest Worldcon yet, and the organizers were very much afraid of getting overwhelmed at the door. One reason was that their guest was Robert A. Heinlein, who hadn't been to a convention since most people active in fandom (remember the 70s were a big growth period) had joined."
"So they did two famous things: They set the at-the-door price at the unheard-of level of $50. And they issued plastic hospital bracelets as proof-of-membership (they also issued name badges, for the purpose of telling people your name)."
Note: attendance was 3,014