Heavy ducks have several indicators, First the butt stock which will have a red rubber Winchester pad. All heavy ducks have a 13 1/2″ length of pull. There will be lead in the butt stock which is visible if you remove one recoil pad screw and loosen the other and turn the pad sideways.

The barrel wall thickness is about double that of a standard model 12 at the muzzle. Heavy ducks all have 3″ chambers and are marked super speed- Super X

If the gun you are looking at meets all these criteria chances are pretty good it’s a real one.

This one has the correct barrel markings. There is a picture of the muzzle which you can see how thick the sidewalls on the barrel are. The condition of the wood does not match the condition of the metal so I would be concerned the butt stock has been replaced with a standard stock. Easily identifiable by the length of pull and if it has lead in it.

Winchester Model 12 3″ Heavy Duck Gun 30″ Original finish NO CC FEE – Pump Action Shotguns at GunBroker.com : 914124756

Hope this helps…

Erin