"Walter," thank you very much for sharing the truly beautiful photographs of your absolutely stunning Ithaca gun. I too think the rare round knob 1/4-grip feature is very pleasing to the eye, just as it is equally mechanically beneficial to its user. The feature helps the gun's employer better control the gun for rough shooting, in my opinion, when compared to one with a straight-grip. On the other hand, I do not like and thus do not use the full-grip, even in, say, a 10-bore gun.
Is Captain Crossman the namesake for the famous surname-branded air rifles, I wonder? Might be interesting to conduct research on the gun's provenance. You have a good start knowing the name of the original owner and the year of his purchase.
You have acquired a real American-made treasure, in every sense of the word; and I am bright green with envy. I think it the nicest high-quality American side-by-side gun I have seen for some time. I had the approximate sense of admiration when viewing the marvelous Remington EE's from about the same era recently on offer at auction.
The reversed choke scheme seems to suggest your acquisition was meant for either driven game shooting or for use against incoming waterfowl. It would be my guess that contemplation of the latter use is the reason for this otherwise odd choke arrangement in an American gun.
My heartiest congratulations to you for your great find and fortunate acquisition! Please let us know how it performs afield and / or before the traps.
Regards,
Edwardian