There have been several MAC 16ga SxS shotguns with these same markings show up from time to time. Heck, even once on this forum:
http://www.thefirearmsforum.com/showthread.php?t=2818
MAC made shooter/hunter grade side-by-side shotguns until 1968. They are not collected as far as I know. If the gun is solid, we can lengthen the chambers for 2 3/4" shotgun shells and modernize the forcing cones.