John Brain (of BikeRodNKustom.com) sent some images of the Swing Bike (then called the Pivicycle) being shown at the 12th International Cycle Show held in New York, March 7-10, 1971! This is the earliest print reference that I've seen on Swing Bikes.
http://swingbike.byethost24.com/bobhufford/earlymagref.html
He also sent in an independent retailer advertisement for Swing Bike from 1976. Check out the reference to Satan ... ;-)
http://swingbike.byethost24.com/bobhufford/76eastgatead.html
Be sure and take a look at John's extensive series of articles covering the early days of the Muscle Bike.
http://bikerodnkustom5.homestead.com/brainhistory58.html
Swing Safely,
Bob Hufford
Springfield, MO