Go down to your local auto glass shop and order it. You should get it the next day. They are still manufactured brand new and are widely availible. Specify convertible, non shaded, w/wo internal antenna.
I purchased a window for a 71 coupe for 130 dollars, I think the convertible was a bit more.
I dont think there are date codes on the windows, but the manufacturer is shown. I'm not into concours stuff so I dont Know what is correct.