I think the most interesting Siatas from a design standpoint were the 208SC barchetta and, especially, the berlinetta. Fairly detailed information on Siata and the berlinetta can be found here and, especially, here. The second source, the Bonhams auction house, has it that the actual designer is unknown, though it has been speculated that it was Giovanni Michelotti.
Siata 208SCs were initially built by Stabilimenti Farina (free translation: Farina Works) that ceased doing business in 1953 after many years as an important Italian coachbuilder. Following the demise, 208 production was continued by Carrozzeria Balbo, another old Italian firm that folded soon after. According to Wikipedia, only 56 208s were built.