Mike: Despite the rib marking, I believe your barrels are Damascus-Twist
David Purnell's 1888 12g 0 grade toplever hammer gun with Damascus-Twist refinished by Dale Edmonds
Gary Carmichael's
Parker 'Plain Twist'
Parker 'Fine English Twist' (which was of course Belgian)
More infro, including Dr Gaddy's Twist samples here
https://docs.google.com/a/damascuskn...LxMESM3W0/edit