I currently own a '66, C-10, Custom and a '66, K-10 Custom and neither one of them have a large back window. I previously owned a '65, C-10, Custom and a '66, C-10, Custom and neither one of them had a large back window.
I read in one of my Chevrolet Truck History books that the large back window was an option. Many truck owners opted for the smaller back glass to help reduce the interior heat and glare, since most trucks from this era were not equiped with factory air conditioning.
I hope this helps put your mind at ease.
