Go to the "ithacagun" dot com home website for parts.
That would be hard to say without a much more detailed description of your Marlins overall condition,to include the amount of original finish remaining on the wood and metal,the bore condition of the barrel etc.
Your serial number indicates that your A.H.Fox shotgun was made in the year 1940.
Assuming you mean an Ithaca double barreled shotgun, there were many diferent models, and they have different values. Some may be $200, some $2000. For any collector's gun, the condition plays a major role in the value. Sorry, you will need a hands on appraisal to get a tru value.