Ithaca began manufacturing shotguns and rifles in 1880, often using expired Remington patents.
Your serial number indicates that your Ithaca shotgun was made in the year 1920.
1976
about 1925
You vave an Ithaca sxs Flues model made in 1911
1938-39
There's a list of serial numbers by year at http://ithacagunrepair.com/page5/page23/
Impossible to answer without the serial number
1924
1905
Impossible to answer without the serial number
It sounds as if you have a Ithaca Crass model shotgun,that was produced in the year 1903 by your serial number.
If this is an old side by side it would be a Baker model made in 1886. Serial numbers of Ithaca shotguns for that year were 2448 - 4101.