It's tough, working blind, but here goes:
What you have is most likely, IMO, a 12ga Savage Model 220 shotgun, that's been reworked for/by someone into a multi-barreled combination set - probably when no such thing was available commercially.
Many Savage M-220 shotguns were made (pre-68) w/o serial numbers, and some not marked at all except for arcane assembly numerals/letters. Look for additional markings inside the action, particularly in the hinge & barrel lug area.
It was once quite common, when both were regular production items as entire arms and as parts, to install Model 219 rifle barrels on the shotgun frame and vice-versa - just as with the H&R Handi, today.
As a matter of fact, Savage even used to offer the M-219 in a rifle/shotgun combination set (rare now).
The (.355/.356) 9mm and the .357 Magnum share bore diameters close enough to allow conversion of a rifled 9mm barrel to the longer, etc, .357 with the proper re-chambering - ergo the caliber remark.
The original, rifled 9mm barrel might have come fron "God know where", but most likely from some equally arcane European rifle, since that caliber was once popular over there, pre-WW II.
The rechambered barrel could have easily been turned to fit a snubbed .410 shotgun barrel stub (which would have had the hinge parts, etc) that got bored out for the barrel to slide in & get soldered/sweated in place permanently.
The "fusil, etc" would be my take on the non-English (probably Italian or Spanish) term for "rifle stock", leading me to believe the so labelled stock might be an imported replacement part/stock.
That's about as close as I can come, by tapping my cane.
YMMV.
.
