The book about Serra's works doesn't clarify totally the question, the cabriolets on SEAT 1400 chassis appeared en 1956 based in the 1400 A and the Seat 1400 B appeared in the same "Feria" as well as the last cabriolet been commercialized by SEAT directly, based in the new chassis. The agreement was that Seat delivers 4 chassis and Serra returned 3 full cars selling the 4th himself as payment for the total work. After 1958 the agreement was cancelled and Serra continued producing cabriolets on order, in such case could be based on an existing chassis from the customer. In any case most of the last cabriolets were based in the 1400 B chassis. So if you doubt to change the A by the B looks more exact to lived as SEAT 1400 Sport Spider by Serra 4th Series.