During its 10 year production the car underwent some changes, and three different models were produced. These were the S1 (1968-1970), the S2 (1970-1972) and the S3 (1972-1978). Each model featured engine power improvements, but only minor details were changed with the exterior design. The interior was altered dramatically between each model. An all new dashboard and steering wheel was installed for the S2, and the interior was again revamped for the S3. In 1970, power assisted steering was offered as an option, and in 1974 an automatic transmission was also offered. In 1976 impact bumpers had to be installed to meet United States safety requirements, and some people consider cars produced with them to be the S4, but Lamborghini did not officially change the designation. Near the end of the Espada's life, Bertone designed a four door prototype, which was never put into production.
In 1999, a new version of the Espada was rumored to be in the works, but it was right at the time Lamborghini wanted to concentrate on a Diablo successor, so little became of the idea aside from a few drawings.
Source: Wikipedia (original)