1968 camaro convertible

Does anyone know why some 68 camaro convertibles have door jamb vents and others do not. I am searching for a nice ride to replace the one I regretably sold a few years back. I have come across several cars and only one of them did not have the door jamb vents. The jambs appeared original, whats up?

Thanks

The car was missing the "cocktail shakers" from the rear quarters, they were still in place in the front fenders. At this point it really doesn't matter the guy sold the car out from under me. I had just never seen one without the vents.
Thanks