The Silver submachine gun is shinier than a normal submachine gun, and also has darker colored wood for the pistol grip and stock. Other than that, it is no different from the normal gun. It has no special stats, properties, or access to any different weapon mods. Weapon mods also get the same color as the original parts.
Sold by KL-E-0 if the silver submachine gun prop is found after the Silver Shroud costume is given to Connolly.
Notes
This weapon cannot be renamed at a workbench, because The Silver submachine gun does not have a unique item ID. The ID 1b28f7 is simply a leveled list the game uses to reward players with the Silver submachine gun during the quest The Silver Shroud. This particular idiosyncrasy is shared by only two other weapons - Ashmaker and the Survivor's Special.
Expansion required
This article is too short to provide more than rudimentary information about the subject. You can help Nukapedia by expanding it.