The UNSC Silberg is a ship of an unknown classification used by ONI for their Radio Beacon Deployment Program. The only known member of the crew is Jason Morelli, a signalman in the Navy and Jersey Morelli's father.
While the classification is unknown, it is possible that the Silberg is an ONI stealth ship, like the Chiroptera-class in First Strike, or something slightly larger, depending on the size of the radio beacons.
The Silberg was absent from The Battle of Reach, as it was on a mission in deep-space.