She's a beautiful gun!
Initially, I thought she was an 871, but she could just as well be a rounded action 557, 570 or 578 that has been engraved in the Model 06 style.
Some uninformed or dare I say--
less than honest sellers have in the past tried to pass off guns such guns as this one as the real deal--an Arrieta Boss Model 06 self-opener that could bring in the neighborhood of $10,000 more for than a 500 series or an 871 gun would. Now having said that, I believe that this gun that you've bought was priced reasonably for what it is.
I'd recommend you go to go to the Arrieta website
www.arrietashotguns.com and decide for yourself what model she is by looking past the engraving and comparing the number and location of the pins.