I don't know too much about Ithaca doubles, but it looks like a Grade 2 NID Model from the engraving and "star". There were several models through the years, Crass, Lewis, Minier, Flues and the NID being the last. Year and serial number would be correct for NID. Value wise, probably $ 7-800.00 I would guess. Period case adds another $ 100 or so. Being your Great Grandfathers makes it absolutely invaluable!! You are very lucky. Hope this helps.