Why do you "believe" it is an Ithaca? It will either have the name "Ithaca" stamped on it somewhere or it won't. If it does, it is, if it doesn't, it isn't.
If the gun is an Ithaca and is from the 1950's as you think, it will not be an SKB and it won't have been made in Japan.
While the advice to not imagine you will get the same money as the gun might fetch in the US is sound, the US prices are still worth knowing to understand what you may have in terms of desirability. Imagining that an early 1950's Ithaca NID SxS should be compared to Stoegers or CZ/Huglus or cheap Spanish guns is lunacy.
A very quick check of Ithaca NID's in relatively original condition in either 28 ga or .410 ga, shows asking prices that range from a low of $4000 to a high of $6500. These guns are rare and valued!
But before you have a hearty attack, imagining you have just won the gun lottery, this is all speculation without photos. If you don't know how to post pics, PM me and I'll post them for you.