While I am no expert, I don't believe that the original 68 SS cars had buckles on the window lift belts. Here is a link to Mancini's old car that is at Mecum now. There are a couple of pics of the interior there. No buckles on the window lift belts on that Dart.

Besides, I tried the buckle. First, it really doesn't do a good job of pulling the window up after the belt goes around all those corners. Second, it is hard on the paint where the belt rubs. And lastly,that buckle flopping around the top of the door when the window is down, isn't cool.

The elastic and a snap is just a better way to go, in my opinion.