Milsurp After Hours

I was planning to also post this in the ID portion. Spanish rimfire double, Eibar Proof and Smokeless powder proof, S50 indicates 1948 date??, imported by Sanderson in Portage, Wisconsin. On bottom of barrels 9m/m45 is stamped in an elongated oval, serial number is AGxxxx, under the fore arm the barrels are stamped 8.3 and 8.3D1. Not sure if Atlas is the maker, or something Sanderson used when marketing the piece. Can't find anything so far on Sanderson. (I got the photos off of another website in which the person was also looking for info and valuing the shotgun, but it is the same as the one I am asking about.) Any help appreciated.

Looks a lot like a double barrel .410 shotgun. The photo sure shows what looks like a rimfire hammer setup which would rule that out. What's the barrel openings look like when you open the breech?