I have seen one other Beristain, a 12 gauge double. It also had some marking in Spanish "Gunmaker to the royal house." I have no idea as to which royal house. The gun wss rather plain, but well made with sling swivels so I assume European origin. I know nothing more about the name. A google search brings up litlle. Regards and best of luck finding out more.

Exorcisms performed cheaply. "We get the Hell out!"