What you have is an Ithaca 1915 Flues model. There are wide variations, but it would top out around 600.00 where I have seen in top-notch shape. An Ithaca freak may cough up considerably more than that, I know i have paid way too much for other people's tastes when I wanted one. That is a firearm that depends on the market you have at hand.
Gun was made in 1915. Does it have damascus or fluid steel barrels? Gauge? Condition is everything... You can get a good idea from the auction sites such as, gunbroker.com or auctionarms.com.
Your serial number indicates that your Ithaca shotgun was made in the year 1920.
It sounds as if you have a Ithaca Crass model shotgun,that was produced in the year 1903 by your serial number.
Impossible to answer without the serial number
1924
It was made in 1919.
1905
about 1925
Impossible to answer without the serial number
5 years
612
With the serial number that you provided,your Ithaca double barrel shotgun with the damascus barrels was produced in the year 1900.