In the scene where Karen pulls the gun on the sleeping Henry Hill the gun changes model numbers multiple times.

As the camera pans and changes from front to rear view the gun changes from a blue steel Model 36 to a stainless steel model 60.

The tip off was on the close up of the hammer. A model 36 is case hardened (a blue merle in color) where as the Model 60 has a stainless (silver color) hammer.