Open the cylinder and read the model number and serial number - stamped on the frame hidden under the crane.
Depending on the model, there will be many variables.... barrel length, sights, trigger, hammer, finish, etc.
For starters, you said it was "black".... if so, it has been refinished. The S&W revolvers of that era were offered as blued (which is very dark and shiny), nickle plated, or, on some models, stainless steel.
12 Gauge: It's also a good idea to include the following:
The Serial Number, on the bottom of the butt-it may have a letter followed by a series of numbers ( The shorter number inside the yoke when the cylinder is open is NOT it-that's an assembly number. Include all numbers and the letter on the butt.
Is the barrel 4" or 6", measured from the absolute end in front of the cylinder to the tip in front of the front sight.
Fixed sights or adjustable - the rear sight would have a small screw to adjust the rear sight for height or left and right, or it would just be a groove along the top strap.
Is the blue worn is places, like around the muzzle, etc; this shows use and will affect the value.
Does it come with the factory box, papers, etc ?
If it does, it may add value.
Please include as much of this stuff as you can-it will help.
It will be interesting to see what you have.