I'm new to Rovers, but an old hand with Jeeps. I don't know enough details on the transfer cases used by the Series vehicles, but you might try reaching out to Herm Tilford. Herm is the man regarding Warn Overdrives used on the Jeep Dana 18 transfer case. I believe he may have worked on or rebuilt Fairey overdrives.
You can find him at www.hermtheoverdriveguy.com