1969. 1968 & 1969 are pretty much the same car (to me) but '68 was the last year Chevelle to have a vent window.
the wing window was offered on the GM A bodys up to 72 if the astro vents were NOT installed in the dash. It's rare to find one or see one on a chevelle. Don't have a production number if any made to back this up but it was offered.
just to prove GM did offer the wing window, here's a 72 Lemans GT-37 without the astro-vents.