Im back after searching almos everywhere.
I have ask at Shotgunworld and arams.es (an Spanish forum)
Not much gain.
Only two clues:
It looks like it was made by BILBATUA Mª LUZ (EUGENIO Y ANTONIO).
There was a stocker named Bereciartua, born in 1912, and as my gun was proofed in 1932, he was 20 years old.
Not much info,
Thanks
Martin

PS: I will post pictures of my gun.