I don't think they were drilled and tapped.
Came from the factory with open sights.
A 20 gauge can be drilled and tapped by any good gunsmith.
Lots of people (myself included) have scoped 20 gauges.
I don't think you'll find a smith that will do the 12 gauge.
It's not thick enough over the chamber to do safely.
I think the new lightweight slug gun is drilled and tapped.
Spanky