Ah, I had completely overlooked your previous reference to Isota Fraschini. While my only reference with a picture indicates that this is a: 1926 Alfa Romeo RL SS by Zagato, since you have already ruled out Zagato I must assume your reference is correct. Using your reminder regarding Isota Fraschini, I have discovered another obscure (at least to me) two name carrozzeria which did an Isotta Fraschini, and thus might be the creator of the mystery Alfa. Is the answer that you are looking for a: 1926 Alfa Romeo RL SS by Carrozzeria Maraga & Tattini?