Aw right, I couldnt see it on the picture. The only other aerial that i can think of would be an old mobile phone aerial. They would be about 12-14 inches long. Do you know much about the history of the car?
Reason i ask is that i used to fit mobile phone systems to customer cars but the cars were mainly owned by businesses as opposed to privately owned. Also, i tended to fit the glass mounted aerial as it was one less hole to have in the bodywork.