I'm quoting myself to update that the serial that was entered in the VIN papers is 8159554488.
I presume the Spanish army decommissioned the truck in the late 70s by adding this WC51 id in the papers taken from the truck where the T214 my '37 it's holding was pulled from. This serial has been stamped in the passenger's side member.
I've sandblasted the chassis and I'm glad finally I could see the REAL serial, from June 1952: 80029888.

Sebas