I can't find any info regarding this car on any sites about Pedro Serra.
Unfortunately I do not have the book written by Pablo Valledor from 2003, maybe it is presented there.
Seems like Carrocero Serra was quite busy in 1955/56 with Pegaso. But they also made
a body for a Seat 1400 A Sport Spider with an almost similar hood in 1956.
So that is my guess - 1956