Start web searching for Winchester shotgun collector boards and similar and I am pretty sure you should find some place that can help you. The 16's used to be a bit less desirable than the 12s or 20s as shooters, but I hear the 16 has made a bit of a comeback. If it is a field grade, I doubt it is worth huge bucks, but with it new and with box and papers, etc. it is certainly a "find". I would place a minimum $1000-1500 value on it for insurance purposes, but I have no idea what it would sell for...maybe not that much or maybe more?

BTW, grand dad had damned good tastes...these are excellent old shotguns and were never "cheap", even 50 years ago.