"Garantizada" translates as "guaranteed (by)" the maker.
"Arenguren" is most likely "Aranguren" (SP) - either Pedro Aranguren or a descendent.
"Plancencia" is a location, Plancencia de las Armas, near Eibar in the Basque region of Spain on the French border - a gunmaking area for centuries, similar to the Gardone/Brescia area of Northern Italy.
You haven't described any smokeless proofmarks, nor the chamber length - which is most likely 2-1/2" (65mm),
So, I would NOT recommend firing ANY modern 20ga ammo in it, should you resolve the lock isues.
(Which is a pretty good reason the lock's been disabled, and to leave it so)