Cervelo has offered several aero road bike frame designs, several of which share the same seatpost design. The carbon, aero-shaped SP20 post is the most used, making it the mostly likely replacement option. See list below for compatibility. In the 350mm length, it is offered in Zero offset and 25mm setback versions, so you have for a more aero position based on a road/TT forward position or to meet proper fit requirements based on femur length and pedal leverage positioning.