The barrels are stub twist.
The original proof marks on the barrels are pre-1875.
It was reproved (nitro proved) 1989-2005 (BAR and bore in mm) prior to importation by Thad Scott Fine Guns in Indianola, MS. Thad was a well respected dealer who died in 2009.
The bores should be 18.2 mm = 0.716 at 9". They were originally .710 - .716.