The Model 12 was Winchester's first hammerless slide action shotgun. There were over 1,900,000 produced. The 16 gauge was made in Standard, Skeet and the special order Pigeon grades. Value will depend on the grade and options.
For the Standard grade, values are: exc-$800, very good-$500, good-$400, fair-$300 and poor-$200. It can have a 26,28,30 or 32" barrel and it can be plain, have a vent or solid rib. The stock is a plain pistol grip walnut.
Skeet grades are double in value with checkered stocks and forends.
Add 20% for solid ribs, 30% for Winchester Special ventilated rib and 40% for milled ribs.Add 20% for 32" barrels. Deduct 10% for 16 gauge.
If you give me the serial number, I can give you the year of manufacture.