I have not seen a 14 bore with 12 gauge chambers before. The reproof stamp may mean the chambers were reamed out from 14 to 12. Maybe there are the telltale,filled-in,little holes at the breech end of the batrrels? Then the pinfire/centerfire conv would be confirmed.
I would be very concerned about the recoil of even soft 12 gauge shells beating the hell out of this gun.