Bit of an amateur here so please bear with me. Clearing out an estate, we've run across this 'Field Grade' Ithaca side by side double barrel shotgun with additional barrel. Serial #'s match on every piece and according to Ithaca site this gun dates to 1935. I'm a bit of a rookie when it comes to these guns and trying to find out all I can so we can get a fair price for it. I believe it's a 16 or 20 gauge (how do you tell for sure?) and appears to be in fairly good condition; a few small rust spots, nick in checkering, a few small dings on barrel. Any info greatly appreciated, ballpark value would be great. I do have more pictures if needed. Thanks for taking the time to look.