Look down the barrel of a KS/X-5 with factory tubes sometime. They flaired out the end to accomidate the tubes. Remington also does thias on their newer contoured barrels. This way they can use thin walled barrels and choke tubes.
They are definately Stan Baker. You can buy a wrench from baker sports to fit the tubes. Sometimes Stan stamped his name on the barrel but not always. Most likely done before he dies as the grandson quit doing the swelled chokes
Stan's early choke tube installations were done using WinChoke tubes. Not sure what he did to choke tube ID but an 870TC he did for me back in late 70's used WinChoke tubes. It was not a BigBore though. Still miss his wit and work.