The 70 had the little nipple on top, in the trunk for the vent hose that ran into the frame rail. Finding a non-vented gas cap that fits is a pain in the butt too because of the shallow cap area on the inlet hole. The depth of the locking ramps and the slot cut-out widths vary too.
A friend left his 70 cap at a gas station on the way to Carlisle 2013. We tried 9 caps from the swap meet before finding one that worked.