Damascus barrel (in French).
A damascus barrel is a barrel made of damascus steel, which is steel produced by hammering two different steels together. The twist refers to the pieces of steel being twisted while they are hammered, which produces the distinctive pattern on the steel.
Impossible to answer without knowing make and model.
With the serial number that you provided,your Ithaca double barrel shotgun with the damascus barrels was produced in the year 1900.
Needs to be examined by a gunsmith
Where did the term derby originate?