Ithaca s/s, 1 1/2, strange?????

My Dad gave me my Grandfathers Ithaca Side by side 12 gauge, marked 1 1/2 in the receiver well. It has 24" barrels, open and modified. There is some minor engraving on the receiver and a trace pattern on the barrel base. It is well checkered, serial # 294,XXX. Over all condition is very good/original. My Granfather bought it back in the late 1930's for a game getter and that it has done. It is the gun I used for years and it shoots great, very, very tight action. Now, first and foremost, I am not a shotgun guy, I know little to nothing about them. I am looking for a little information on the gun that may help me understand a little more about it, I am really perplexed about the 1 1/2, Is it a model or something? any help would be appreciated-SDH:sos:

I think that is known as a Flues model, 1 1/2 is the frame size. If the barrels are 24" original length, never altered, the gun was a special order.