The World's First Cabriolet Fighter

Open cars may have a folding roof but only detachable sidescreens which snap-on, they do not convert into a fully enclosed car with proper weather-protection. The wind-up windows make the distinction between an open car and an all-weather car now known as a convertible.
Convertibles are a luxury because they are more expensive to build than closed cars, the matching sedans or saloons. Open cars were significantly cheaper to build and very much lighter than sedans, saloons or convertibles and usually very flexible.
Open cars often have or had four doors but with the later emphasis on rigidity of construction four-door convertibles became extremely rare.