I've seen them both ways. I have a few factory photos of 69's with the single bar. One theory was that early production cars used the single bar, then sometime very early in the production run, they switched to the crosshair style. Some additional theories were that the a/c cars had the single bar, while non-a/c had the crosshairs. But several examples (unmolested cars) were given that were contrary to these theories. Perhaps one plant had extra single bar vent balls left over from the 68 run, and used them at their discretion. I doubt anyone will ever know.

Factory photos aside, I would definately use the crosshair (+) style! I consider these "correct" for the 69's.