Your serial number indicates that your Ithaca shotgun was made in the year 1920.
That is a phone number, not a serial number. Ithaca did not make 3 billion shotguns.
It sounds as if you have a Ithaca Crass model shotgun,that was produced in the year 1903 by your serial number.
It was made in 1919.
Impossible to answer without the serial number
With the serial number that you provided,your Ithaca double barrel shotgun with the damascus barrels was produced in the year 1900.
It sounds to me that you have a Ithaca Flues model double barrel shotgun.With the serial number that you provided,your shotgun was made by Ithaca in 1917.
Need to know model