Butterfly he means end fastener those are 8020 parts
Bar 1 has the butterfly screwed into the hole running down the center by tapping the hole
Bar 1 is attached to bar 2 by sliding the butterfly in like a tnut and tightening it down through a hole in the bar called an access hole
See the second page of here
http://download.8020.net/2011_Produc...Catalog_17.pdf