Without more ID info, it's going to be tough to identify just which model it is.
Ithaca made a number of double guns over the years.
Here's a link to Gun Parts Corporation. They sell many parts for the various Ithaca doubles, and possibly you can ID which model it is with the exploded views.
Click on the views to expand them.
Best guess is, it's a Model 100, 200E, 280, 0r 400.
All these are basically the same gun so stocks may interchange.
If everything else fails, contact Gun Parts for help in IDing it. You can also send them the stock you have as a sample.
Here's other sources of Ithaca parts: