I am by no means knowledgeable regarding this detail, so what follows is speculation.
In the images below, you will notice that the convertible tops have rectangular, removable panels that house the small backlights. Sometimes, convertibles would been driven with their tops up and the panels gone, perhaps to provide better ventilation on non-rainy days. The reason for the small windows (and here I speculate) is that they were made of glass or a stiff piece of clear plastic for good vision to the rear. Glass is heavy, and the canvas tops were not strong enough to support large windows. So convertible backlights had to be small if they were glass or a heavy plasitc. The downside to this is that small windows greatly restricted the driver's rear view.
Later convertibles tended to feature larger backlights made of thinner transparent plastics.
The following images are of cars offered for sale, usually at auctions. Presumably, their convertible tops are of authentic design, even though they might be replacements for worn out originals. I cannot guarantee authenticity in all cases, however. That said, what is striking is how similar the backlights are for so many brands over so many years.