1953 Chevrolet rear vent windows?

I need some car history buffs to help me out here. I didn't think 1953 Chevrolet rear vent windows existed. My 1954 Pontiac has rear vent windows, and I thought they were Pontiac exclusive.



But then I was watching this old advertisement for the 1953 Chevrolet, and sure enough, I spotted the Pontiac-style rear vent windows in the 4-door demo model.



Interestingly enough, these vent windows also appear in the Chevy brochure for that year.




Before I came across this YouTube video, I had never seen a '53-'54 Chevrolet with rear vent windows. So what's the deal? Were they an obscure option? Was this feature changed before the car reached market?

 

Stop the video at 11:57 and you'll see a disclaimer stating "Rear door ventipanes shown on Four Door Sedans are not available." This must have been a last-minute change before production. So unless a '53 sedan prototype turns up, it looks like there aren't any like that around.

 

Oh wow, there it is. I didn't make it that far into the video.