Since you ain't gettin any love on this Q I will start out with this: I would think with the tough fed gov't emission regs of that era (starting in '68 in most states, maybe 67 in California, not sure exactly) that it came with a non vented cap. gas fumes would be a major polluter. I think the C.A.P. (C.A.S)? system started in '68. I know my 68 440 GTX had the valve cover breather connected to the air cleaner as opposed to being open. Not sure how the tank is vented for the fluid to flow out/forward