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.[1]
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.
Sources
Partial transcript of On Reach.wav
Jersey: Durga is my dad on reach!?... Durga: He wasn't on Reach... Jersey: Thank god. Durga: ...He's on deep space reconnaissance on a ship called the Silberg. Jersey: Whew!